Near-IR tunable laser with an integrated LiTaO3 electro-optic deflector Joanna L. Casson, Li Wang, Nathaniel J. C. Libatique, Ravinder K. Jain, David A. Scrymgeour, Venkatraman Gopalan, Kevin T. Gahagan, Robert K. Sander, and Jeanne M. Robinson
We report the successful demonstration of a near-IR tunable laser 共1525.4 –1558.2 nm兲 that uses an integrated LiTaO3 deflector in combination with a reflection grating as an electronically tunable filter. The electro-optic deflector is a unique integrated optical device and consists of a horn-shaped array of electro-optic prisms in series. The almost 33 nm of tuning covers a wavelength region of high interest to the communications industry 共1527–1550 nm兲. © 2002 Optical Society of America OCIS codes: 140.0140, 140.3600, 230.0230, 230.2090, 230.3120, 160.2260.
1. Introduction
Rapidly tunable lasers are in great demand for wavelength division multiplexed optical communications because of the need to transport information over multiple wavelength channels on a single fiber. The lasers need to be reliable and low in cost. These lasers can either be used as spares for fixedwavelength lasers in case of failure, as tunable sources in advanced wavelength-routed optical networks, or as high-performance lasers for component testing and characterization. There is also a need for such rapidly tunable lasers in IR spectroscopy and for detection of toxic chemicals. A tuning mechanism that is nonmechanical, electronically tuned, and has the potential to quickly scan accurately and repeatably over a wavelength range would be desirable. One popular approach to tunable laser design involves use of a gain medium in an external-cavity architecture with wavelength tunability provided by an intracavity tunable filter. Most current tunable filter designs have moving parts such as motor-driven J. L. Casson, L. Wang, N. J. C. Libatique, and R. K. Jain are with the Center for High Technology Materials, University of New Mexico, Albuquerque, New Mexico 87106. D. A. Scrymgeour and V. Gopalan are with the Pennsylvania State University, University Park, Pennsylvania 16802. K. T. Gahagan is with Corning, Incorporated, Corning, New York 14831. J. L. Casson 共
[email protected]兲, R. K. Sander and J. M. Robinson are with the Division of Chemistry, Los Alamos National Laboratory, Los Alamos, New Mexico 87545. Received 28 June 2002; revised manuscript received 7 August 2002. 0003-6935兾02兾306416-04$15.00兾0 © 2002 Optical Society of America 6416
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rotatable mirrors,1 prisms or reflection gratings,2 or piezoelectrically tunable fiber Fabry–Perot filters,3 although nonmechanical filters such as acousto-optic deflectors4 have also been demonstrated. However, most of these approaches1–3 suffer from backlash and hysteresis and do not have the advantages of the electronic tunability of the laser presented here, whereas acousto-optic deflectors4 normally require high rf drive powers and are difficult to design with narrow 共⬍1 nm兲 passbands. A nonmechanical tuning mechanism, such as electro-optic tuning, that allows rapid, accurate, and reproducible wavelength tuning is highly desirable. We report the successful demonstration of a near-IR tunable laser 共1525.4 –1558.2 nm兲 that uses an electro-optic tuning mechanism based on a new tunable filter by use of an integrated LiTaO3 deflector in combination with a reflection grating. The electro-optic deflector design5 is based on a series of inverted prism domains of monotonically increasing dimensions in a horn-shaped geometry for maximum angular deflection range, in contrast with a rectangular geometry based on a series of equally sized inverted prism domains6 共Fig. 1兲. The 15-mm-long, 286-m-wide horn-shaped deflector, designed in such a way that each successive prism is just wide enough to accommodate the deflected beam, is capable of a deflection of 27.2 mrad兾kV at 1558 nm.5,7 For a rectangular deflector and horn-shaped deflectors of equal length and comparable widths, a higher voltage can be applied to the horn-shaped deflector than the rectangular deflector, before the beam is deflected out of the side of the device. We chose to implement our deflectors in LiTaO3 because the near-constant domain-wall velocities during the poling of these
Fig. 1. Diagram showing the domain boundaries of the hornshaped deflector.
Fig. 3. Laser wavelength as a linear function of applied voltage. The slope of the line is 5.9 nm兾kV.
crystals provide greater control of prism dimensions, making it easier to fabricate these devices with high yield.8 Although LiNbO3 has a slightly higher index of refraction and larger electro-optic coefficients than LiTaO3, the relatively jerky8 domain-wall motion during poling of LiNbO3 makes it more difficult to implement optimized horn-shaped deflector geometries in this material. Subsequent to the experiments described here, we became aware of a recent conference proceeding in which a 780-nm laser diode was tuned with an electro-optic deflector by use of a tuning concept similar to that reported here.6 However, because LiNbO3 was used, the deflector design was implemented in a rectangular geometry and, as discussed above, resulted in an electronic tuning range of only 1 nm, in contrast to the nearly 33 nm of tuning presented here with the horn-shaped scanner design.
ished to avoid lasing off that end, and a 20⫻ microscope objective was used to collimate the fiber output. A 75-mm focal-length lens was used to focus the collimated beam into the electro-optic deflector, whereas an 84-mm focal-length aplanat lens was used to near collimate the output beam from the deflector onto the fixed grating. Applying a voltage to the device deflects the light beam and consequently changes the angle of incidence onto the grating. The wavelength-dependent angular sensitivity, combined with the angular deflection from the electro-optic deflector, acts as the wavelength tuning mechanism. A maximum applied voltage of 4.3 kV 共15.9 kV兾mm兲 was used to remain well below the coercive field of ⬃21 kV兾mm used to create the domains in the deflector. It was necessary to tilt the deflector slightly to avoid lasing off the front face of the crystal.
2. Experimental Setup
3. Results and Discussion
The schematic diagram of the near-IR tunable laser is shown in Fig. 2. The gain medium used was a 980-nm diode-pumped Er:SiO2 fiber. The laser cavity was formed by a gold-coated fiber tip at one end of the cavity and an external gold-coated reflection grating 共600 grooves兾mm, 28° 41⬘ blaze at 1500 nm, 78% efficiency兲 positioned after a collimating lens. The fiber facing the scanner and grating was angle pol-
A.
The grating filter passband ⌬ is determined by ⌬ ⫽ 兾N, where is the wavelength of the laser and N is the number of grooves illuminated on the 600groove兾mm grating. The laser intracavity beam, incident on the grating, was measured to be 1.1 mm in diameter, resulting in an estimated passband of ⬃2.5 nm. This calculation is in agreement with the measured passband of 2.4 ⫾ 0.1 nm. B.
Fig. 2. Schematic showing tunable near-IR laser with an electrooptic deflector. HR, high reflectance, WDM, wavelength division multiplexing, f.l., focal length.
Passband
Laser Results
Figure 3 illustrates the 32.8-nm scanning range that we observed with this laser when the applied voltage was tuned from ⫺1.5 to 4.0 kV. The wavelength of the laser was a linear function of the applied voltage 共5.9 nm兾kV兲. The accuracy of the wavelength selection is dependent only on the accuracy of the voltage source. Figure 4 shows a set of spectra with a slightly less optimum deflector alignment than the previous set, showing a total tuning range of 31.9 nm and a 5.3-nm兾kV tuning efficiency. Tuning gaps at ⫺1.5 and ⫺0.5 kV were observed. The power loss as the beam propagated through the ferroelectric crystal at these voltages was probably too high for lasing to occur. The loss was likely a result of the laser being 20 October 2002 兾 Vol. 41, No. 30 兾 APPLIED OPTICS
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Fig. 4. Spectrum analyzer data at 12 applied voltages for a less optimum configuration than Fig. 2.
misaligned with the ferroelectric domains and scattering occurring at the domain walls. The laser could be tuned over its entire range by more careful alignment of the laser with the ferroelectric domains. With a pump power of 150 mW, the laser power was 200 W at a wavelength of 1558 nm. The ratio of the peak power to the background ranges from ⬃15.5 to almost 30 dB. For most of the wavelength range, the ratio is greater than 20 dB, which is sufficient for many laser applications. However, the ratio of the peak power to the peak 共at 1534 nm兲 of the amplified spontaneous emission varies from a low of 2 dB to a maximum of 19.5 dB, with the majority well above 10 dB. The resolution bandwidth of the optical spectrum analyzer used was 0.1 nm, whereas the laser linewidth is 2.4 nm. The amplified spontaneous emission peak can be suppressed by the addition of an external or intracavity gain-flattening filter. C.
Optimum Lens Position
The intracavity beam’s angle of incidence onto the grating determines the wavelength supported in the laser cavity.9 Ideally, the beam must be collimated to ensure good coupling of the reflected light back into the fiber. However, to the first order, if the beam is perfectly collimated, angular deflection at the scanner will merely be translated into linear displacement as the beam exits the aplanat lens, resulting in zero tuning efficiencies. Therefore, in the present laser embodiment, a design trade-off exists between the filter’s tuning efficiency and effective insertion losses. To find the optimum lens position, we varied the tuning efficiency from 5 to ⫺4.3 kV兾nm by moving the lens from ⫺11 to 5 mm, where the origin corresponds to the aplanat lens positioned one focal length from the focused beam waist inside the LiTaO3 deflector. These tuning efficiencies were found to be consistent with first-order estimates based on a simple model.9 4. Future Directions
There are several key improvements that could be made to the laser to increase its tunable range, narrow the passband, or make it more compact. One such improvement would be to antireflection coat the crystal faces to allow the deflector to be perfectly 6418
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aligned with the output of the fiber laser, enabling use of the full deflection range of the device 共27.2 mrad兾kV兲,7 corresponding to an improvement of 50%. With filter linewidths estimated to be 2.5 nm, the laser was expected to have a multimode emission over a ⬃0.25-nm spectral range. Note that the focus of these experiments was on the demonstration of an effective electronic tuning mechanism for tunable near-IR fiber lasers and not on the generation of narrow-linewidth emission. However, future tunable lasers can be designed for single-frequency operation by use of denser grating grooves, grazing incidence 共Littman–Metcalf configuration10兲 geometries, appropriately designed secondary filters 共e.g., high-finesse etalons兲, or linewidth-narrowing devices 共e.g., wavelength-tracking saturable absorbers11兲. The tunable laser can be made more compact by use of gain media with significant gain anisotropy 共e.g., quantum-well semiconducting materials or rareearth-doped polarization-maintaining fibers兲 and rotated to match the crystal’s optimum polarization orientation to eliminate the intracavity polarizer and by use of a device with a collimating electro-optic lens train and a horn-shaped deflector prism train integrated in a single device, similar to one such electrooptic element that we have previously reported,12 eliminating two intracavity lenses. Although the current design of the tunable laser requires kilovolt voltages, by using thinner devices 共of the order of tens of micrometers rather than hundreds兲 or using materials with higher electro-optic coefficients, we could reduce the applied voltages to a more reasonable tens of volts. We can also make the laser more versatile by using gain media that emit at different spectral ranges. Future tunable lasers can also be designed to emit at different spectral ranges within the optical transmission window of LiTaO3 共0.4 –5 m兲. Experiments are currently in progress to demonstrate a tunable mid-infrared 共3-m兲 fiber laser with diodepumped Er:ZBLAN fibers.13 5. Conclusion
We have demonstrated purely electronic wavelength tuning of a near-IR laser that incorporates an electrooptic deflector, eliminating the need for a motordriven grating. The design resulted in ⬃33-nm electronic tuning range that exceeds that of the previously reported 共⬃1-nm electronic tuning ranges兲, similarly designed visible laser.6 Because the electro-optic effect is a virtually instantaneous nonlinear optical phenomenon, the device speeds are limited only by the electrode design 共i.e., drive and input impedance matching, optical and rf driving field coincidence—traveling-wave design issues兲, and the switching times can be in time scales of nanoseconds to picoseconds, allowing the potential for use in a large number of applications in which both wavelength tunability and tuning speed are important, such as in reconfigurable wavelength division multiplexing sources14 or spectroscopic uses15 such as differential absorption lidar.
This project was funded through the Laboratory Directed Research and Development program at Los Alamos National Laboratory. D. A. Scrymgeour and V. Gopalan also acknowledge support from the National Science Foundation grant 9988685. References 1. B. Pezeshki, “New approaches to laser tuning,” Opt. Photon. News 12, 34 –38 共2001兲. 2. N. J. C. Libatique, J. Tafoya, N. K. Viswanathan, R. K. Jain, and A. Cable, “Field-usable diode-pumped ⬃120 nm wavelength-tunable CW mid-IR fibre laser,” Electron. Lett. 36, 791–792 共2000兲. 3. J. L. Zyskind, J. W. Sulhoff, J. Stone, D. J. DiGiovanni, L. W. Stulz, H. M. Presby, A. Piccirilli, and P. E. Pramayon, “Electrically tunable, diode-pumped erbium-doped fiber ring laser with fiber Fabry–Perot etalon,” Electron. Lett. 27, 1950 –1951 共1991兲. 4. V. I. Kravchenko, V. M. Moskalev, Yu. L. Oboznenko, Yu. D. Opanasyuk, E. N. Smirnov, and V. V. Taranov, “Tunable laser with an in-resonator acoustooptic deflector,” Pis’ma Zh. Tekh. Fiz. 8, 174 –178 共1982兲. 5. J. C. Fang, M. J. Kawas, J. Zou, V. Gopalan, T. E. Schlessinger, and D. D. Stancil, “Shape-optimized electro-optic beam scanners: experiment,” IEEE Photon. Technol. Lett. 11, 66 – 68 共1999兲. 6. M. Laschek, M. Reich, D. Wandt, W. Arens, C. Fallnich, and H. Welling, “External-cavity diode laser with electrooptic wavelength tuning,” in Conference on Lasers and Electro-Optics 共CLEO兾US兲, 1999 OSA Technical Digest Service 共Optical Society of America, Washington, D.C., 1999兲, pp. 141–142. 7. K. T. Gahagan, J. L. Casson, D. A. Scrymgeour, R. K. Jain, J. M. Robinson, and V. Gopalan, “A large angle electro-optic
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