Diffused Ti:sapphire channel-waveguide lasers - OSA Publishing

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J. Opt. Soc. Am. B / Vol. 21, No. 8 / August 2004

Hickey et al.

Diffused Ti:sapphire channel-waveguide lasers Louise M. B. Hickey,* Vasilis Apostolopoulos, Robert W. Eason, and James S. Wilkinson† Optoelectronics Research Centre, University of Southampton, Southampton, UK

Andrew A. Anderson Department of Physics, University of Southampton, Southampton, UK Received May 23, 2003; revised manuscript received February 6, 2004; accepted February 11, 2004 The fabrication and operation of Ti:sapphire channel-waveguide lasers is presented, in which both the gain medium and the waveguide are formed by the thermal diffusion of titanium. Lasing was observed between wavelengths of 775 nm and 821 nm, with the lowest launched pump-power threshold being 210 ⫾ 40 mW for a pump wavelength of 514.5 nm. © 2004 Optical Society of America OCIS codes: 130.3120, 140.3590.

1. INTRODUCTION A miniature broadly tunable waveguide laser would present an elegant light source for numerous applications in sensing, spectroscopic, and diagnostic instrumentation. Ease of mass manufacture and robustness of construction are essential for applications in large-volume portable equipment, and the planar-waveguide configuration offers a platform for integrating wavelength-selection and tuning components onto the same chip, to realize a robust, multifunctional, low-maintenance device. Research into tunable integrated optical lasers in dielectric media has until recently concentrated on rare-earthdoped crystals and glasses.1–3 Titanium-doped sapphire presents an ideal laser material for broadly tunable miniature lasers, with a broad gain band leading to an output spectrum that is continuously tunable over more than 400 nm, from 650 nm to 1100 nm.4 Such a laser, based on an homogeneously doped Ti:sapphire crystal, was first demonstrated in 19825 and rapidly became commercially successful. In this configuration, the pump-power thresholds are typically between 1 and 2 W4 at wavelengths near the peak of the gain, so high-power sources are necessary to pump the laser, limiting the application of the laser outside the specialized laboratory. In the research environment, pump-power thresholds as low as 100 mW have been achieved with a short, heavily doped crystal in a resonator with a small beam waist.6 Integrating a Ti:sapphire laser into a planar-waveguide configuration has the potential for further reducing the pump-power threshold, as high intensities may be achieved for low powers and maintained over the length of the waveguide. The lack of divergence of the pump and laser modes also allows a longer cavity with a lower Ti3⫹ concentration to be employed, improving design flexibility. Achieving thresholds of the order of tens of milliwatts is key to realizing practical low-cost, small footprint lasers with viable pump sources, such as frequencydoubled solid-state sources, or blue-green diodes when they become readily available with sufficient output powers and lifetimes. The compact planar format offers the 0740-3224/2004/081452-11$15.00

potential for combining intracavity wavelength-selection and tuning components in an integrated optical circuit on the same sapphire chip, enhancing functionality and ease of use. Furthermore, sapphire is a versatile substrate for the growth of ZnO for the generation of acoustic waves,7 potentially leading to methods for producing short modelocked pulses and for the growth of GaN for generation of short-wavelength radiation,8 leading to the realization of an integrated pump source. Thus the realization of a Ti:sapphire laser in a waveguide geometry may lead to an attractive compact portable laser, broadly tunable over 400 nm and with the potential to produce short modelocked pulses, opening up a host of new applications. Interest in miniature waveguiding Ti:sapphire lasers has accelerated in recent years. Approaches have included a novel melt, diffusion, and recrystallization technique,9 the growth of doped aluminum oxide layers by metal–organic plasma-enhanced chemical vapor deposition,10 the implantation of Ti into sapphire11 following earlier implantation studies,12,13 the growth of crystal fibers, the growth of Ti:sapphire layers by pulsed-laser deposition,14–17 and the approach of the present authors of Ti-diffusion into sapphire.18 The latter three techniques, to our knowledge, have led to the realization of waveguide lasers. The first of these was the Ti:sapphire crystal-fiber laser reported in 1988,19 with subsequent development in 1995.20 In the latter paper, 11.2-mm-long crystals of diameter 350 ␮m were used as a gain medium and multimode waveguide. The laser was pumped by a frequency-doubled Nd:YAG laser and was set up with an external cavity that included a birefringent filter for tuning. The laser was tunable over 56.5 nm, which corresponds to the free spectral range of the tuning element, and the power performance was limited by the spectroscopic quality and geometric uniformity of the crystal fiber. Crystalline layers grown by pulsed-laser deposition on a sapphire substrate led to the realization of a planarwaveguide laser21 shortly after the first Ti-diffused laser was reported.22 The 12-␮m-thick crystalline layers supported a waveguide mode in the depth dimension, leading © 2004 Optical Society of America

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to laser operation with an absorbed pump-power threshold of 560 mW with an output mirror having less than 2% transmission for a 4-mm-long cavity. A maximum output power of 357 mW was achieved for a 35% transmission output mirror and 3.7 W of absorbed power, corresponding to a slope efficiency of 26%. Operation typically used a 5% pump duty cycle, leading to quasi-continuous-wave (cw) operation, although true cw operation was observed for output powers up to 73 mW with a 5% transmission output mirror, when convective cooling was used to stabilize the device temperature. The first Ti-diffused waveguide lasers were formed in a channel waveguide realized by diffusion of Ti from a stripe source.22 In common with the other lasers described above, the Ti-doped region forms both the optical waveguide and the gain medium. Laser operation was achieved in a 6-mm-long channel waveguide with a threshold pump power of 1.2 ⫾ 0.4 W, assuming a 41 ⫾ 12% coupling efficiency into the waveguide, with a 2% transmission output mirror. High round-trip intracavity losses were reported and attributed to poor contact between the waveguide and cavity mirrors. In this work, the route of thermal-diffusion doping has been pursued further due to the versatility of the doping process, clearly demonstrated by the success of the numerous diffusion-doped LiNbO3 devices.23 By thermal diffusion, complex channel-waveguide circuits may be readily formed by prior definition of the diffusion source using standard photolithographic techniques. In addition, more than one dopant may be introduced in a series of fabrication steps allowing precise control of the device composition. Such control of the composition by diffusion doping has enabled the evolution of complex active and passive devices and will provide a route to forming the envisaged multifunctional laser. In this paper, the power characteristics and spectral properties of a diffused Ti:sapphire channel-waveguide laser are presented, with an emphasis on achieving low pump-power threshold. Trends in the pump-power threshold are investigated as a function of pump wavelength and pump duty cycle, leading to observations of pump-power thresholds as low as 210 ⫾ 40 mW for a pump wavelength of 514.5 nm. This threshold is less than a fifth of what we reported in Ref. 22, showing promise for low-power pumping with solid-state sources.

2. TI:SAPPHIRE WAVEGUIDE LASER MODEL The spectroscopy of uniformly doped bulk titaniumsapphire crystals has been extensively studied.24–29 The material is characterized by a broad absorption at wavelengths in the blue-green and a broad fluorescence band extending from 650 nm to 1100 nm. The optical transitions are due to electronic transitions of the outermost electron 关 3d 1 兴 on the Ti3⫹ ions, which are substitutionally incorporated on the sapphire lattice. Under the influence of the crystal field, the 3d electron energy levels split into two distinct bands that are broadened due to coupling with vibrations in the lattice. Optical transitions between the bands may occur, with rapid nonradiative decay between vibrational modes, leading to an approxima-

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tion to a four-level laser system. Excess absorption losses at wavelengths near the peak of the gain band at 760 nm have been reported and attributed to the disruption of the crystal field due to a combination of defects and the presence of Ti4⫹ ions.27–29 Such absorption losses are described by a figure of merit (FOM), defined as the ratio of the absorption at a wavelength of 500 nm to that at a wavelength of 800 nm. Early crystals were characterized by FOMs of less than 100, with the loss by reabsorption limiting the performance of the laser; however, commercial Ti:sapphire crystals are now realized with FOMs in excess of 500.30 In the waveguide configuration, the performance of the Ti:sapphire laser can be described approximately by a simple model. The waveguide geometry is shown in Fig. 1, where the direction of propagation is parallel to the z axis, with the pump and signal radiation confined in the x and y directions by a channel waveguide. Since, in the work presented here, both the waveguide and the active medium are formed by titanium ions diffused into the sapphire substrate, the distribution of the Ti3⫹ is assumed to vary in the x and y directions and remain invariant in the z direction. This geometry and analysis is specific to the Ti:sapphire system discussed here, but follows similar principles to previously presented four-level laser systems with reabsorption loss.31 In thermal equilibrium, for low population inversion, the excited-state population is proportional to the incident pump intensity, I p (x, y, z), the density of groundstate ions, N 0 f(x, y), the fluorescence lifetime, ␶, the quantum efficiency, ␩ q , the pump-absorption cross section, ␴ p , and 1/photon energy (1/h ␯ p ), where h is Planck’s constant and ␯ p is the pump frequency. This relationship is given in Eq. (1), where N 0 is the number of active ions per unit length, and f(x, y) is the distribution of the active ion in the x and y directions, normalized so that 兰兰 f(x, y)dxdy ⫽ 1 per unit area: N 共 x, y, z 兲 ⫽

␶ N 0␴ p␩ q h␯p

I p 共 x, y, z 兲 f 共 x, y 兲 .

(1)

The intensity of the pump in the waveguide, I p , is related to the pump power by I p (x, y, z) ⫽ P p (z)S p (x, y), where P p (z) is the pump power and S p (x, y) is the normalized mode distribution, such that 兰兰 S p (x, y)dxdy ⫽ 1 per unit area. At the signal wavelength, the change in power along the waveguide is related to the population inversion, the stimulated-emission cross section, ␴ s , and the intensity of radiation at the signal wavelength, I s (x, y, z) ⫽ P s (z)S s (x, y), where P s (z) is the signal power and

Fig. 1.

Ti:sapphire waveguide laser model geometry.

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S s (x, y) is the mode distribution at the signal wavelength, normalized so that 兰兰 S s (x, y)dxdy ⫽ 1 per unit area. The power at the signal wavelength is attenuated by the propagation loss in the waveguide at the signal wavelength, ␤ s , and by reabsorption defined by the FOM. This relationship is described by Eq. (2), as follows: dP s 共 z 兲

⫽

dz

冕冕

N 共 x, y, z 兲 P s 共 z 兲 S s 共 x, y 兲 ␴ s dxdy

冉

⫺ ␤s ⫹

N 0␴ p FOM

冊

P s共 z 兲 .

(2)

For laser operation, the round-trip gain must equal the losses at the cavity mirrors that have power reflectivities, R 1 and R 2 , contributing a loss coefficient ln(R1R2). Therefore the pump power necessary to achieve laser threshold may be calculated from Eq. (2), yielding the relationship given in Eq. (3) below, in which ␤ p is the propagation loss at the pump wavelength and L is the waveguide length: P th ⫽

冋冉

␤s ⫹

⫻ ⫻

N 0␴ p FOM

冊

L⫺

ln共 R 1 R 2 兲 2

册

N 0␴ p ⫹ ␤ p N 0 ␴ p 兵 1 ⫺ exp关 ⫺共 N 0 ␴ p ⫹ ␤ p 兲 L 兴其

冕冕

1 S s 共 x, y 兲 S p 共 x, y 兲 f 共 x, y 兲 dxdy

h␷p

␶ ␩ q␴ s

.

(3) This expression serves to illustrate the influence of the most significant physical parameters of the Ti:sapphire waveguide laser system upon the predicted laser pump threshold. Additional cavity losses may occur in an experimental device, for example, due to misalignment of the cavity mirrors or due to poor quality of the polished waveguide end face. In addition, the dependence of spectroscopic properties on the physical environment is not made explicit. For example, the excited-state lifetime is affected by temperature,26 and the measured quantum efficiency varies with the spectroscopic quality, which is often linked to the Ti concentration and the fabrication process.32 The first group of terms in Eq. (3) describes the relationship between pump-power threshold and the losses in the cavity at the signal wavelength. For a waveguide laser, propagation losses are likely to be significant, at ⬃1 dB/cm for first-generation devices, perhaps reducing to the order of 0.1 dB/cm with refinement of the fabrication process, in a similar way that Ti:LiNbO3 waveguide losses have been reduced since their first demonstration.33 In comparison, the reabsorption loss at the signal wavelength, defined by the term N 0 ␴ p /FOM, is likely to be small. For example, at typical doping levels of 0.1 wt.% Ti2 O3 in Al2 O3 (3.3 ⫻ 1019 atoms/cm3 ) with ␴ p ⫽ 9.3 ⫻ 10⫺20 cm2 (Ref. 24), the absorption will be the equivalent of 0.13 dB/cm for a FOM of 100. This value scales accordingly, increasing to 0.26 dB/cm for a FOM of 50 or for a doping level of 0.2 wt.% Ti2 O3 in Al2 O3 .

Therefore in contrast to the case of the bulk Ti:sapphire laser, the FOM will not be the major performance-limiting factor if reasonable quality material can be realized, until waveguide propagation losses are below 0.1 dB/cm. The third group of terms in Eq. (3) describes the overlap of the waveguide mode at the pump and signal wavelengths and their interaction with the distribution of the active ion. As the form of each function S s (x, y), S p (x, y), and f(x, y) becomes similar, the overlap integral increases. Further, as the x, y space occupied by the functions decreases, for example, as the waveguide modes become smaller, the numerical values necessarily increase, leading to significant increases in the integral if a good overlap is maintained. Therefore the pump-power threshold is highly sensitive to the components of the overlap integral, which are controlled by the characteristics of the diffused waveguide. In a device where a single dopant controls the waveguide refractive-index profile and defines the distribution of the gain medium, this term will provide a limitation on the performance of the laser. The predicted pump-power threshold of a representative waveguide laser is plotted against waveguide length in Fig. 2, using Eq. (3) in conjunction with a simple model for the waveguide mode profiles. The modes are assumed to have Gaussian distributions in the x and y dimensions at the pump and signal wavelengths, with the dimensions approximated to those measured for the experimental laser reported below. At the pump wavelength, the full width of half-maximum intensity spot size is taken to be 4 ␮m ⫻ 7 ␮m, and at the signal wavelength 5 ␮m ⫻ 11 ␮m, in the x and y dimensions, respectively, corresponding to experimental values reported in Subsection 4.C. The pump absorption along the length of the waveguide is taken to be that corresponding to Al2 O3 uniformly doped with 0.12 wt.% Ti2 O3 . The spectroscopic properties are taken from previously published data, with ␴ p ⫽ 9.3 ⫻ 10⫺20 cm2 , 24 ␴ s ⫽ 3.8 ⫻ 10⫺19cm2 , 34 and ␶ ⫽ 3.2 ⫻ 10⫺6 s. 25 The published range of quantum efficiencies at 300 K is from 0.42 to 0.8,25–27,35 depending on the doping level and the growth conditions, and is taken to be 0.5 for the purposes of this illustration. The pump and signal wavelengths are assumed to be 500 nm and 800 nm respectively, and the cavity is formed by two mir-

Fig. 2. Estimated pump-power threshold for (a) no propagation losses or reabsorption losses, (b) 1-dB/cm propagation loss at the pump and signal wavelengths, (c) 1-dB/cm propagation loss at the pump and signal wavelengths, and reabsorption losses equivalent to a FOM of 50, (d) 3-dB/cm propagation loss at the pump and signal wavelengths, and (e) 5-dB/cm propagation loss at the pump and signal wavelengths.

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rors with power reflectivities of 0.9996 at the signal wavelength. A high mirror reflectivity has been chosen for this illustration as it corresponds to those used in experiments reported below to show low-threshold operation. Clearly such high reflectivities will lead to poor slope efficiency, and the output mirror must ultimately be optimized for a specific application. The estimated pump-power threshold in the absence of any propagation or reabsorption losses is given in Fig. 2, curve (a), showing that, for this ideal case, the threshold becomes submilliwatt as the device length increases and the pump is absorbed. With propagation losses of 1 dB/cm at the pump and signal wavelengths, Fig. 2, curve (b), shows the pump-power threshold increases, yielding a minimum of 43 mW for an optimum length of just under 0.5 mm. For a 4-mm device length, the threshold pump power is almost double at 77 mW. With the inclusion of reabsorption losses corresponding to a FOM of 50, the threshold increases by ⬃30% to 100 mW for a 4-mm device, as shown in Fig. 2, curve (c). With increasing waveguide propagation losses of 3 dB/cm and 5 dB/cm, the pump-power threshold increases and the optimum length decreases as illustrated in Fig. 2, curves (d) and (e). Above threshold, the laser output power increases with increasing pump power with a slope efficiency related to the efficiency of coupling out of the cavity and the total round-trip cavity loss. This slope efficiency, ␩ s , is given in Eq. (4) below, where it is assumed that the pump radiation is completely absorbed in the cavity

␩s ⫽

␭p ␭s

␩q

T 共T ⫹ ␦兲

f ovl ,

(4)

where T is the total transmission through the cavity mirrors, ␩ q is the quantum efficiency, f ovl represents the fraction of absorbed pump photons converted to signal photons, including geometric and signal reabsorption factors,36 and (T ⫹ ␦ ) is the total fractional round-trip loss at the laser wavelength, given in Eq. (5).

冋冉

共 T ⫹ ␦ 兲 ⫽ 1 ⫺ R 1 R 2 exp ⫺2L ␤ s ⫹

N 0␴ p FOM

冊册

.

(5)

For a given round-trip loss, the coupling out of the cavity, T, may be optimized for a desired output-power performance, balancing the slope efficiency with the laser threshold. The launched pump-power slope efficiency will be lower than ␩ s by a factor given by the fraction of launched pump power absorbed by Ti3⫹ ions in the cavity,

冠冉

N 0␴ p N 0␴ p ⫹ ␤ p

冊

冡

兵 1 ⫺ exp关 ⫺共 N 0 ␴ p ⫹ ␤ p 兲 L 兴其 .

Using the waveguide model outlined above, f ovl ⬎ 0.7, increasing to ⬎0.95 well above threshold considering a channel-waveguide laser pumped and lasing in the fundamental mode with a FOM contributing less than 0.5 fractional round-trip loss.36 For f ovl ⫽ 0.8, the slope efficiency with respect to launched pump power would be 0.07%, 0.03%, and 0.02% for a 1-dB, 3-dB, and 5-dB excess round-trip loss, for the high-reflector mirrors employed. The numerical example above considers a high-Q cavity, with a poor quantum efficiency and short absorption

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length. A lower-Q cavity would enhance the slope efficiency, while increasing the pump-power threshold. With higher round-trip cavity losses, the optimum length would advance into the millimeter regime as the threshold increased. In a similar manner, a reduction in the Ti3⫹ concentration and increase in absorption length would lead to longer optimum lengths and an increase in the pump-power threshold. The effect of higher pumppower thresholds could be addressed by refinement of the waveguide design with a reduction of the modal spot size, or an increase in the modal overlap between the pump and signal wavelengths. While employing many assumptions, the numerical example given here clearly illustrates the effect of cavity length, propagation loss, and the FOM on the pumppower threshold. Low-threshold devices are readily achievable in the waveguide geometry with reasonable materials, waveguide, and cavity parameters.

3. EXPERIMENTAL PROCEDURES A. Ti:Sapphire Waveguide Laser Fabrication Ti-diffused sapphire waveguides were fabricated by diffusing stripes of titanium oxide into a commercially available sapphire wafer. The sapphire wafer, approximately 0.3 mm thick, was supplied by Union Carbide30 with one face polished to high optical quality and oriented with the plane face perpendicular to the c axis. A 10 mm ⫻ 10 mm sample was cut from the sapphire wafer and cleaned in a series of organic solvents and a mixture of hydrogen peroxide and sulfuric acid. Standard photolithographic techniques were used to define a series of channel openings of width 4.5 ␮m ⫾ 0.5 ␮m in a photoresist film. A film, 270 ⫾ 7 nm thick, was thermally evaporated from a Ti2 O3 powdered source onto the masked substrate, in a low partial pressure of oxygen (1 ⫻ 10⫺4 mbar). It is expected that a mixture of titanium oxide phases will exist in the evaporated film.37 With the photoresist mask subsequently removed in acetone, a series of stripes of deposited titanium oxide of width 4.5 ␮m ⫾ 0.5 ␮m remained on the surface. The sample was then placed in the center of a carbon resistance furnace, in a continuous flow of dry purified argon. The furnace was switched in less than 1 min to the diffusion temperature of 1700 ⫾ 60 °C and left for a period of 1 h. The furnace was then switched off so that the sample cooled rapidly to room temperature in the argon atmosphere. Separate measurements showed that the furnace temperature stabilizes within 10 min of switch-on and cools to room temperature within 10 min of switch-off. In previous work, the cooling rate has been found to affect the proportion of diffused Ti in the fluorescent Ti3⫹ state, with a significant enhancement in the Ti3⫹ fluorescence yield resulting from rapid cooling.18 After diffusion, the sample was cut to leave a 4-mmlong device, and the opposite end faces were polished parallel to high optical quality. For the laser experiments, mirrors were attached to the end faces to form a laser cavity. The mirrors were supplied by Melles Griot and consisted of a multilayer dielectric stack on a thin glass slide. At a wavelength of 500 nm, the coatings had a transmission of approximately 0.9, and at wavelengths between

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760 nm and 810 nm, the coating reflectivity was 0.9996 ⫾ 0.0002. The mirrors were attached by first supporting them with a small volume of fluorinated oil, Fluorinert FC-70 (3M), to hold the mirror close to the sample by surface tension. The mirrors were then fixed more rigidly using a small volume of acetone-soluble adhesive. The sample was left overnight to ensure that the Fluorinert evaporated and was then inspected under an optical microscope to ensure good contact between the reflective coating and the waveguide end face. B. Measurement of Effective Dopant Concentration and Spectral Attenuation Light from an argon-ion laser was coupled into the TM00 mode of a representative waveguide, so that the E field was parallel to the sapphire c axis (␲ polarized). The incident and transmitted power was measured at wavelengths of 457 nm, 466 nm, 472 nm, 476 nm, 488 nm, 501 nm, and 514.5 nm. The average modal concentration of Ti3⫹ was estimated by comparing the total loss through the waveguide at these wavelengths with published data for the absorption of Ti3⫹ in sapphire. Over this wavelength range, the absorption spectrum for a Ti:sapphire crystal doped with 0.09 wt.% Ti2 O3 (3 ⫻ 1019 Ti atoms/cm3 ) is well known, and the absorption coefficient ranges from 2.04 ⫾ 0.06 cm⫺1 to 2.89 ⫾ 0.06 cm⫺1 . 25 By this method, the absorption coefficient at each wavelength can be separated from the waveguide propagation loss and the insertion loss if it is assumed that, over this small wavelength range, only the absorption coefficient changes. C. Laser Power Characteristics Two Ti-diffused channel waveguides were investigated for laser operation, and these are referred to below as laser 1 and laser 2. Characterization of the waveguide lasers was carried out using the apparatus shown in Fig. 3. Linearly polarized light from an argon-ion laser operating at a wavelength of 476 nm, 488 nm, 496 nm, or 514.5 nm, or on all lines simultaneously, was directed toward the sample, passed through a mechanical chopper, and focused into the waveguide through the polished end face using a 6.3⫻ microscope objective lens. The pump radiation was aligned to excite a TM (␲ polarized) waveguide mode, as above. The chopper blade and speed were adjusted to achieve a typical pump on:off time of 1:19 ms. Radiation was collected from the waveguide output with a 10⫻ microscope objective lens, and unabsorbed pump radiation was separated from wavelengths greater

Hickey et al.

than 700 nm using a reflective short-wavelength filter (cold mirror). The radiation transmitted by the cold mirror was directed toward either a single detector placed in position 1 or two detectors located in positions 1 and 2. This configuration enabled measurement of the laser output power to ensure optimum coupling of the pump radiation, while simultaneously monitoring waveguide mode properties or spectral characteristics. A long-wavelength pass filter, absorbing wavelengths shorter than 610 nm, was used to block any residual pump radiation transmitted through the cold mirror. A broadband pass filter with transmission greater than 68% over the wavelength range 770 nm to 830 nm was used to block any fluorescence outside the reflection band of the cavity mirrors. A neutral-density filter with optical density 2.0 was used to prevent saturation of the detector. The transmission of all optical components in the path between the receiver and the waveguide output was measured to enable the output power exiting the waveguide laser to be determined. A calibrated silicon avalanche photodiode (Theoptics APD50) and a digital storage oscilloscope were used to measure and record the waveguide laser output power. Measurements were made with a microsecond resolution throughout the pump interval, and statistics were gathered for average power and standard deviation received over a number of pump intervals. D. Waveguide Mode Intensity Profiles The waveguide mode intensity profiles were recorded using a silicon CCD camera (Pulnix PE2015) in position 1 (Fig. 3), with the silicon avalanche photodiode receiver in position 2. The pump-laser wavelength was selected, and pump coupling into the waveguide was optimized for maximum laser output power. The magnification of the imaging system was calibrated by moving an illuminated graticule into the place of the waveguide. Correct focusing of the waveguide output end face was ensured by prior illumination with a white-light source. The mode intensity profiles were recorded for both the unabsorbed pump power and the output laser radiation, using appropriate filters before the CCD camera. E. Laser Output Spectra Spectral measurements were carried out using an optical spectrum analyzer (OMA 2000 triple-grating spectrometer, EG &G Princeton Applied Research Model 1471A) in position 1. The silicon avalanche photodiode receiver was placed in position 2 to allow simultaneous measurement of the output power. Spectra were recorded over a 1-s integration time to yield average laser spectra over fifty 1-ms pump intervals, with resolution better than 0.12 nm.

4. EXPERIMENTAL RESULTS

Fig. 3. Experimental configuration for characterization of the waveguide lasers.

A. Effective Dopant Concentration The average Ti3⫹ concentration distributed over the TM00 mode was measured to be 0.12 ⫾ 0.02 wt.% Ti2 O3 in Al2 O3 , calculated from the total absorption in a representative waveguide at wavelengths near 500 nm. This is at the high end of the range used for commercial Ti:sapphire lasers.

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B. Laser Power Characteristics Output power is plotted against launched pump power (pump power coupled into the waveguide input) for lasers 1 and 2 in Fig. 4, for a pump wavelength of 514.5 nm. The output power was recorded for one hundred 1-ms pump intervals at each pump power to give averages, and the standard deviations are shown in Fig. 4 as error bars. The launched pump power was calculated from the pumplaser output power, using the measured transmission of the optical components and assuming a 50% pumpcoupling efficiency into the waveguide, an estimate based on prior measurement with an equivalent optical system. The largest source of error in launched power will arise from the estimate of the coupling efficiency into the waveguide, such that a ⫾10% error in this estimate will lead to an error of ⫾40 mW in the laser threshold. For laser 1, the measured threshold pump power is 210 ⫾ 40 mW, while for laser 2, the threshold is 220 ⫾ 40 mW. The characteristics of both laser 1 and 2 show a repeatable deviation from linear behavior above threshold. In addition, both lasers show regions of increased power instability. These observations indicate changes in the cavity configuration and gain medium both from pump cycle to pump cycle and with pump power. For pump powers up to 350 mW, the slope efficiencies relative to launched pump power are 0.11% and 0.08% for lasers 1 and 2, respectively, assuming equal bidirectional operation from the symmetrical cavity. Referring to Eq. (4), and taking ␭ p ⫽ 500 nm, ␭ s ⫽ 800 nm, ␩ q ⫽ 0.5, the total power transmission out of the cavity to be T ⫽ 0.0008 and f ovl ⫽ 0.8, the total round-trip cavity loss is estimated to be 0.6 dB and 0.9 dB for lasers 1 and 2, respectively. Laser operation was also achieved with the argon-ion laser pump source operating at wavelengths of 476-nm, 488-nm, and 496-nm and in a multiline configuration. The power characteristics for laser 2 are presented in Fig. 5 for a pump wavelength of 488 nm and for multiline pumping. The threshold pump power in the case of multiline pumping is greater than that observed for the 514.5-nm pump wavelength and increases further for the pump wavelength of 488 nm. Figure 6 shows the pumppower threshold for different pump wavelengths, showing that the threshold increases with shorter pump wavelengths. In all cases, the threshold obtained for laser 1 is

Fig. 4. Power characteristics for waveguide lasers pumped at a wavelength of 514.5 nm.


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Fig. 5. Power characteristics of laser 2, pumped with an argonion laser operating multiline and at a wavelength of 488 nm.

Fig. 6. Pump-power threshold for pump wavelengths of 476 nm, 488 nm, 496 nm, and 514.5 nm.

lower than that obtained for laser 2, indicating consistently higher cavity losses or poorer pump/signal overlap for the latter. To investigate the potential for continuous-wave operation, laser 2 was pumped at 514.5 nm, while the chopper blade and the chopping frequency were varied to change the pump on:off durations. First, the pump on:off ratio was varied, maintaining the pump interval constant at 1 ms, to yield on:off times of 1:19 ms, 1:9 ms, 1:3 ms, and finally, 1:1 ms. Secondly, the pump chopping frequency was lowered to give on:off time of 2:2 ms, 10:10 ms, and 100:100 ms. Figure 7 shows the pump thresholds measured under these conditions. The thresholds were 225 ⫾ 40 mW for pump ‘‘off ’’ durations longer than the ‘‘on’’ duration, with a significant increase in threshold to 270 ⫾ 40 mW for on:off ratios of unity. Under conditions of continuous pumping, no laser operation was observed for launched pump powers up to 490 mW.

C. Waveguide Mode Intensity Profiles The waveguide mode intensity profiles were measured for laser 1, operating at an output power just above threshold for pump wavelengths of 514.5 nm, 488 nm, and 476 nm, and are shown in Fig. 8. Figure 8(a) shows the intensity

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distribution of the laser radiation, which is characteristic of a fundamental mode, TM00 . The full size (depth and breadth) at half peak intensity is 5 ⫻ 11 ␮m. Figure 8(b) shows the waveguide mode profile at a pump wavelength of 514.5 nm, the wavelength that yields the lowest pumppower threshold. This distribution is characteristic of a TM00 mode, with a full size at half peak intensity of 4 ⫻ 7 ␮m. At the shorter pump wavelengths of 488 nm and 476 nm, Figs. 8(c) and 8(d) show that the pump-coupling conditions lead to excitation of modes other than TM00 , when the pump coupling is optimized for maximum output power. The altered pump intensity distribution will affect the overlap integral between the pump radiation, signal radiation, and the Ti3⫹ distribution. A decrease in the overlap integral at the shorter pump wavelength is consistent with the higher pump-power thresholds shown in Fig. 6.

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Fig. 9. Normalized emission spectra for laser 1 operating with a pump wavelength of 514.5 nm, for launched pump powers in the range from 250 mW to 1050 mW.

Fig. 10. Normalized emission spectra for laser 2 operating with a pump wavelength of 514.5 nm, for launched pump powers in the range from 250 mW to 850 mW.

Fig. 7. Threshold pump power for laser 2 operating with various pump duty cycles and a pump wavelength of 514.5 nm.

Fig. 8. Waveguide mode profiles for laser 1: (a) Ti:sapphire laser radiation; (b), (c) and (d) pump radiation at wavelengths of 514.5 nm, 488 nm, and 476 nm, respectively. Scale: full width at half-maximum of mode shown in (a) is 5 ⫻ 11 ␮m.

D. Laser Output Spectra Figures 9 and 10 show the laser output spectra for lasers 1 and 2, respectively, for a range of launched pump powers and with a pump wavelength of 514.5 nm. Just above threshold, laser 1 operates at wavelengths between 780 nm and 783 nm. At higher pump powers, this laser operates over a broader band, extending to wavelengths between 775 nm and 805 nm. However, the spectral coverage is incomplete under the pump conditions used, with no lasing observed between 786 nm and 800 nm. The spectral properties of laser 2 are shown in Fig. 10. Just above threshold, the laser operates at wavelengths between 795 and 797 nm. With increasing pump power, the spectrum broadens with emission observed between 785 nm and 805 nm. Laser 2 emits over the full spectral range for the conditions presented, in contrast with laser 1. Despite being nominally the same, the two lasers show very different output spectra. It is believed that the spectral behavior is dominated by Fabry–Perot etalons formed between the mirrors and the waveguide end faces, or from small variations in the mirror reflectivity, which are not repeatable from device to device. Future integration of waveguide reflection gratings or coupled cavities is expected to eliminate this problem. Temporal changes in the emission spectra during the 1-ms pump interval were investigated for laser 2 using a series of 10-nm bandpass filters, centered at wavelengths of 790 nm, 800 nm, and 810 nm. Using each filter in

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turn, the output power was captured for a single pump interval using the silicon avalanche photodiode and digital storage oscilloscope. The oscilloscope was triggered by the chopper signal, and triggering conditions were not altered between measurements so that the waveforms could be overlaid and compared. The power output of the laser during the pump interval, in each of the three wavelength bands, is shown in Fig. 11, for the laser operating with a multiline pump at a launched pump power of 1260 mW. Throughout the 1-ms pump interval, the power transmitted through the 790 nm, 800 nm, and 810 nm bandpass filters varies strongly. Similar variations were observed for lower pump powers and for pump wavelengths of 488 nm and 514.5 nm. This wavelength switching during laser emission indicates temporal changes in the gain and loss within the laser cavity during pumping. E. Variation of Laser Output Spectrum with Pumping Conditions Figure 12 shows the emission spectra for laser 2 operating with a pump on:off time of 100:100 ms for pump pow-

Fig. 11. Temporal behavior of laser 2 power output when pumped with an argon-ion laser operating multiline. The launched pump power is 1260 mW.

Fig. 12. Emission spectra for laser 2 operating with a pump of wavelength 514.5 nm and 100:100-ms pump on:off time, for increasing pump powers above threshold, (a) 30 mW above threshold, (b) 100 mW above threshold, and (c) 280 mW above threshold.


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ers 30 mW, 100 mW, and 280 mW above threshold. Just above threshold, the laser operates at wavelengths between 803 nm and 818 nm, with the spectrum broadening to 821 nm at a pump power 100 mW above threshold. With 280 mW of pump power above threshold, the spectrum shifts to lower wavelengths, with emission between 792 and 809 nm. These spectral characteristics contrast with those presented in Fig. 9 for a 1:19 ms on:off pump interval, which shows the laser operating at wavelengths between 786 and 805 nm. The sensitivity of the spectral characteristics to changes in pump conditions illustrates the delicate balance that exists between gain and loss in the cavity. It is expected that heating and heat dissipation will affect the cavity dynamics and may adversely affect the spectroscopic properties of the material, potentially preventing the laser from operating in a continuous mode.

5. DISCUSSION While the combination of threshold and slope efficiency drives the overall economics of a laser system, a low threshold presents the opportunity for a greater choice of pump laser. For example, realizing thresholds of the order of milliwatts would enable the use of a compact semiconductor diode source, when available at a suitable wavelength. The move to a pump laser with lower overall power consumption and a smaller footprint would then enable the realization of a portable Ti:sapphire laser system. Low thresholds are achievable in a bulk Ti:sapphire laser by using a short heavily doped crystal and a tightly focused pump and signal beam waist.6 The high dopant concentration ensures that the pump is strongly absorbed in the short length for which the spot size remains small and the pump/signal overlap remains high. An optical fiber or waveguide geometry allows small spot sizes and high overlap integrals to be maintained over the entire length of the waveguide, which lead to low thresholds and relaxes the requirement for high Ti3⫹ concentration, if the waveguide losses are sufficiently low. Ti:sapphire crystal fiber lasers reported to date exhibit a core diameter of 350 ␮m,20 compared with the diffused planar waveguides reported here that have a mode diameter of less than 10 ␮m, so that lower pump-power thresholds may be expected for the latter due to the decrease in the cross-sectional area of the active waveguide medium. While the circular symmetry of the crystal-fiber lasers may have benefits in some applications, the planar geometry offers greater design flexibility and presents a route to integration of tuning and control elements within the same chip. An effective Ti2 O3 concentration of 0.12 wt.% was measured in the diffused waveguides, representing an average over the waveguide mode. This doping level would be considered high for a commercial laser system, but is governed by the need to use the same dopant to provide the gain and to increase the refractive index and thereby form a waveguide. Following standard diffusion theory, the highest Ti concentration will occur at the surface, decreasing with depth into the substrate, such that the maximum Ti concentration will be greater than the measured modal average of 0.12 wt.%. At the high surface

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concentrations, it is likely that the spectroscopy of the material is compromised. Furthermore, to achieve more tightly confined waveguides, higher maximum concentrations are required, as this concentration of titanium is expected to yield an increase in refractive index of order 1 ⫻ 10⫺4 above the substrate index, to form the waveguide.38,39 Therefore while it is projected that sub100-mW pump-power thresholds may be readily achieved with waveguide lasers formed solely by the diffusion of titanium, the requirement for high Ti concentrations may raise some performance barriers by compromising the spectroscopic quality of the material due, for example, to the increased influence of Ti3⫹ – Ti4⫹ pairs and resultant fall in material figure of merit.40 Two waveguide lasers have been realized, with launched pump-power thresholds of 210 ⫾ 40 and 220 ⫾ 40 mW and slope efficiencies of 0.11% and 0.08%, respectively. The round-trip loss in each 4-mm-long cavity is believed to be dominated by waveguide propagation loss and mirror butt-coupling loss and is estimated from Eq. (4) to be 0.6 dB and 0.9 dB, respectively. From these figures, upper estimates of the distributed waveguide loss at the signal wavelength are 0.8 dB/cm and 1.1 dB/cm. These estimates assume a quantum efficiency of 0.5 and would be higher if the quantum efficiency is greater. The loss estimate is sensitive to errors in the reflectivity of the cavity mirrors, such that an error of ⫾0.0002 would give an error in the round-trip cavity loss of ⫾0.5 dB. In comparison, the model for a Ti:sapphire waveguide laser described in Section 2 shows that, for a similar configuration and 1 dB/cm loss at the pump and signal wavelengths, the expected pump-power threshold is 77 mW, less than half the measured value. The underestimate of the threshold may arise from overestimates of the quantum efficiency, lifetime (as discussed below), and the overlap between the pump and signal modes. These errors, combined with errors in the estimate of the roundtrip loss, are sufficient to explain the discrepancy between the measured threshold and the expected pump-power threshold. The observed pump-power threshold is significantly lower than previous reports for a titaniumdiffused sapphire waveguide laser22 and is less than half that reported for the Ti:sapphire waveguide laser formed by pulsed-laser deposition.21 The improved performance compared with the earlier diffused waveguide lasers is due to a significant reduction in intracavity losses, as better proximity between the cavity mirrors and waveguide end face has been achieved. Further reduction in intracavity losses and a reduction in the modal spot size are likely to result in further decrease in the threshold pump power to below 50 mW. For example, direct deposition of cavity mirrors and the realization of buried or clad waveguides could reduce the intracavity loss. A reduction in the mode size could be achieved by increasing the Ti concentration; however, this will also affect the pump absorption and optimum device length. Therefore higher numerical-aperture waveguides may be better achieved by introducing a second ion to independently form a passive-waveguiding medium. Such a passive ion requires careful choice to ensure the spectroscopy of the Ti and the crystalline structure of the sapphire is not compromised by the additional dopant. For example, gal-

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lium indiffusion has led to the demonstration of waveguides in sapphire, although the compatibility with Ti-doping is yet to be determined.41 The lasers operated in a quasi-cw mode for a wide range of pump duty cycles, although true cw operation was not achieved with continuous pumping. This compares with early demonstrations of the bulk Ti:sapphire laser, where the pump radiation was pulsed or chopped to attain laser operation at room temperature.25 The initial failure to obtain cw operation was attributed to heating in the sapphire and the poor spectroscopic quality of early crystals. With a rise in temperature, the proportion of nonradiative decay increases so that both the fluorescence lifetime and the quantum efficiency decrease.26 Furthermore, at lower temperatures, sapphire exhibits better thermal conductivity, enabling rapid heat dissipation. True cw operation was first achieved by cooling the Ti:sapphire crystal to liquid-nitrogen temperatures.25 Ti:sapphire lasers were subsequently demonstrated at room temperature as the quality of the available material increased, although cooling was used to enhance heat dissipation from the active region.27 Water cooling of the Ti:sapphire remains a feature of commercial lasers. Similar effects may be prohibiting true cw operation in the waveguide lasers reported here. Even with unity quantum efficiency, the difference between the pump and signal wavelengths causes approximately 40% of the incident energy to be dissipated nonradiatively, resulting in heating of the active medium that will impair the laser operation in the absence of adequate thermal management. Reduced quantum efficiency will exacerbate this problem. Sapphire has a positive coefficient of refractive index with temperature, and a ⌬n of approximately 1.5 ⫻ 10⫺4 results from a 10 °C rise in temperature.42–45 This index change is similar to that which forms the Tidiffused waveguide. Localized heating may therefore be expected to lead to significant changes in the waveguide geometry, which would alter the waveguide mode profile and its overlap with the Ti concentration, and adversely affect the spectroscopy of the Ti3⫹ ion in sapphire. Such thermally induced variations, in addition to thermomechanical changes in the pump input coupling, intracavity loss, and internal power distribution, are likely to cause the deviations in linearity of power output shown in Figs. 4 and 5. Thermal management and temperature stabilization of the waveguide laser, perhaps incorporating convective, water, or Peltier cooling of the wafer, needs to be considered in future generations of the Ti:sapphire waveguide laser. Cw lasing has been observed for the pulsedlaser deposition waveguide laser with convective cooling,21 and it is anticipated that similar results may be achieved for the diffused waveguide with improved thermal management. Laser emission was observed over a wavelength range of almost 50 nm, between 775 nm and 821 nm, with the spectral range dependent upon the pump power and duty cycle, for example. Temporal wavelength instability was evident, with uncontrolled switching between emission wavelengths. In a commercial laser system, a birefringent filter introduces a wavelength-dependent cavity loss yielding a stable laser wavelength with a linewidth of ⬍30 GHz.4 Incorporation of an equivalent wavelength-

Hickey et al.

selection component, such as a grating, or a multicavity structure, on the chip within the waveguide laser cavity will be necessary to enhance the spectral stability of the Ti-diffused waveguide laser. Thermal diffusion is a convenient method to introduce dopants locally into a substrate to realize waveguides or regions of gain. Channel waveguides are realizable in a single-step manufacturing process by diffusion, and additional dopants may be added, either in the same or a subsequent diffusion step, to enhance functionality. In particular, the integration of passive components, for instance, for intracavity wavelength selection, is likely to require waveguide sections with and without the active ion. Such multidopant devices are manufacturable with a layer-deposition process, but this would require dopedfilm deposition and removal by ion-beam milling and subsequent undoped-film deposition and patterning, for example. For the ideal integrated optical Ti:sapphire laser, the characteristics of the gain medium and the formation of the waveguide need to be decoupled, so that the waveguide index and the Ti concentration may be independently selected for optimum performance. This has been successfully achieved for multifunctional rare-earthbased laser systems in LiNbO3 . 23 However, in comparison to rare-earth dopants, the spectroscopy of the transition metal ion, Ti, in the sapphire host is extremely sensitive to the local crystal field.46 For example, in the presence of Fe2⫹, Ti will be included preferentially as Ti4⫹, and the crystal will have the well-known blue color of the sapphire gemstone.47 However, careful selection of a compatible codopant and optimization of control over the diffusion conditions is expected to lead to a second ion that may be included in the sapphire to form an independent waveguide.41


*Now with Southampton Photonics Inc., Southampton, UK. † E-mail: [email protected].

REFERENCES AND NOTES 1.

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3. 4. 5. 6.

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10.

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6. CONCLUSIONS Channel-waveguide Ti-diffused sapphire lasers have been realized, where the diffused titanium forms both the gain medium and the optical waveguide. With a pump wavelength of 514.5 nm, the lasers operated with a pumppower threshold of 210 ⫾ 40 mW and 220 ⫾ 40 mW and slope efficiencies of 0.11 and 0.08%. The output wavelengths ranged from 775 nm to 821 nm for pump powers up to 1050 mW under varying pump conditions. In all instances, the lasers were operating in a quasi-cw mode. These results illustrate that Ti:sapphire waveguide lasers, with thresholds appropriate for an all-solid state system, are attainable by thermal diffusion of titanium into sapphire.

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ACKNOWLEDGMENTS The authors would like to thank D. P. Shepherd, D. A. Sager, B. J. Ault, and P. Hua for helpful discussions and technical support. This work was supported in part by The Royal Commission for the Exhibition of 1851 with the provision of a Fellowship. The Optoelectronics Research Center is an Interdisciplinary Research Center supported by the UK Engineering and Physical Sciences Research Council.

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