Design, Characterization, and Applications of Index ... - IEEE Xplore

IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 17, NO. 6, NOVEMBER/DECEMBER 2011

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Design, Characterization, and Applications of Index-Patterned Fabry–P´erot Lasers Stephen O’Brien, Frank Smyth, Member, IEEE, Kai Shi, John O’Carroll, Prince M. Anandarajah, David Bitauld, Simon Osborne, Richard Phelan, Brian Kelly, James O’Gorman, Frank H. Peters, Member, IEEE, Brendan Roycroft, Brian Corbett, and Liam P. Barry, Senior Member, IEEE (Invited Paper)

Abstract—The design and measured performance characteristics of a range of index-patterned diode laser sources are presented. These devices incorporate slotted regions etched into the laser ridge waveguide, which are formed in the same fabrication step as the ridge, thus avoiding the requirement for complex lithography and regrowth steps. We first demonstrate that the index profile of single and multimode devices can be obtained directly from an inverse problem solution based on a perturbative calculation of the threshold gain of the longitudinal modes of the cavity. Measurements of temperature stability, linewidth, and modulation bandwidth of single-mode devices obtained in this way are presented. It is then shown that the design of multimode devices including two-color and pulsed mode-locked devices designed to support a discrete comb of modes is also possible. We finally demonstrate a tunable source based on a multisection design defined using etched features. This device is shown to have wide tunability with narrow linewidth modes and fast wavelength switching speed. Index Terms—Laser resonators, mode-locking, semiconductor lasers, tunable lasers.

I. INTRODUCTION VER increasing demand for bandwidth is expected to drive a large market in consumer optical communication equipment in the coming years. Applications including fiber to the home and 10-Gigabit Ethernet require laser transmitters that offer excellent performance, including wide bandwidth, stable single-mode emission, and tolerance to temperature variation

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Manuscript received December 8, 2010; revised February 10, 2011; accepted February 13, 2011. Date of publication April 25, 2011; date of current version December 7, 2011. S. O’Brien, D. Bitauld, S. Osborne, B. Roycroft, F. H. Peters, and B. Corbett are with Tyndall National Institute, University College Cork, Cork, Ireland (e-mail: [email protected]; [email protected]; simon. [email protected]; [email protected]; [email protected]; [email protected]). F. Smyth, K. Shi, J. O’Carroll, P. M. Anandarajah and L. P. Barry are with the Rince Institute, Dublin City University, Dublin 9, Ireland (e-mail: [email protected]; [email protected]; john.ocarroll@eblanaphotonics. com; [email protected]; [email protected]). R. Phelan and B. Kelly are with Eblana Photonics, Ltd., Dublin 2, Ireland (e-mail: [email protected]; brian.kelly@eblanaphotonics. com). J. O’Gorman was with Eblana Photonics and is now with Xylophone Optics (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSTQE.2011.2118192

and optical feedback. These performance characteristics must also be achievable at very low cost [1]. Demand for devices that provide spectrally pure singlemode emission is complemented by increasing interest in multimode and tunable diode laser sources. Among the diverse applications of multimode devices, we can identify terahertz (THz) generation by photomixing, short-pulse generation by mode-locking and all-optical switching [2]–[4]. Fast wavelength tunable devices on the other hand are a key component in many proposed wavelength agile networks and optical interconnects. These optical-based schemes may become an alternative to electronic switching in future telecommunication and high-performance computing networks. Their development also follows from the demand for increased bandwidth but may also provide greater throughput with lower latency as well as lower power consumption [5]–[7]. While semiconductor Fabry–P´erot (FP) lasers are very convenient to manufacture, the FP geometry is unsuitable for many applications because of the lack of any frequency selectivity other than that provided by the gain medium. The gain bandwidth in typical semiconductor lasers is much larger than the FP mode spacing, with the result that the lasing spectrum of an FP diode laser will generally comprise many longitudinal modes. The intensities of these modes vary according to the dispersion of the material gain, which in turn can strongly depend on the drive parameters. In order to overcome the limitations of plain FP devices, many types of single-mode laser diodes have been reported including distributed feedback (DFB) and distributed Bragg reflector (DBR) lasers as well as coupled cavity and external cavity lasers [8]. Of these examples, the most commonly deployed is the DFB laser. Translational symmetry of the grating profile determines the lasing modes of these devices, without the need for a reflection from external mirrors [9]. Although these devices have been in use for some 30 years, predictable and high-yield fabrication remains a challenge due to the requirement of highresolution lithography involving two or more epitaxial growth steps. Variations of grating-based approaches have also provided examples of multiwavelength devices. Examples of so-called two-color devices include devices based on DFB and on distributed Bragg reflections [10], [11]. However, it is not clear if these design approaches can address the problem of mode

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selection where greater than two modes with predetermined wavelengths and mode spacings are required. A wide variety of widely tunable lasers with excellent performance characteristics have also been demonstrated in recent years. One possible approach is based on arrayed single-mode lasers [12], while true multiwavelength tunable designs typically use some form of Bragg grating and multisection structure to define the modal structure and tuning mechanism. Modern devices of this kind can span as many as 100 channels or an entire communications band with great accuracy. While wide and accurate tunability is a significant advantage for many applications, these devices are complex to manufacture and they may suffer from broad optical linewidth. On the other hand, tunable devices based on external cavities may provide narrow linewidth emission, but these devices typically have a large footprint and relatively slow switching times. The consumer optical communications industry requires high-performance single-mode lasers in the long wavelengths that can be reliably manufactured with high yield and low cost. For this purpose, an alternative approach to DFB lasers is the index-patterned or slotted laser, where slots etched into the ridge waveguide of a diode laser can provide modal selection. The slots, formed in the same etch step as the ridge, are deep enough to perturb the modal structure but do not penetrate the active region [14]. Consistency in the slot reflection is important and is achieved through careful management of the etching processes in order to achieve reproducible and uniform feature shapes. An etch stop layer is used to ensure uniform depth and shape to the slot features. This simple fabrication process requires no epitaxial regrowth or high tolerance lithography, and hence, fabrication yields can be high offering the potential for high-volume, low-cost fabrication [1]. It has been shown that once the output spectrum of the device is specified, the required distribution of slots along the laser cavity can also be derived directly from an inverse problem solution [14], [15]. The significance of this result follows from the fact that the design of multimode index-patterned lasers also becomes possible. In this paper, we will present the design and characterization of various index-patterned FP lasers. We first describe a design approach that enables the precise tailoring of the lasing mode spectrum in a single-section FP device. The design and characteristics of single-mode, two-color, and passively mode-locked index-patterned lasers based on the slot etching technique will be presented. We show finally that the slot etching technique can be extended to multisection devices to provide discrete tunability with fast switching speed but with potentially lower cost and complexity than commercial tunable laser diodes. This paper is organized as follows. In Section II, we present a brief description of our theoretical approach and the associated inverse problem solution. The application of slot etching techniques is illustrated using the example of a single-mode device. The performance characteristics of single-mode indexpatterned devices are presented in Section III. Measurements of linewidth, side-mode suppression ratio (SMSR), modulation

Fig. 1. Refractive index profile of an FP laser of length Lc . The cavity effective index is n1 and the N additional index step features have effective index n2 . The feedback in the laser is provided primarily by the external mirrors r1 and r2 . The matrix T relates the left and right moving fields in the cavity at the cavity mirrors as shown.

bandwidth, and temperature performance are presented. In Section IV, we describe the design and applications of two multimode devices. The first is a two-color laser with a primary mode spacing in the THz region. The second is a passively mode-locked device. The design and performance of a tunable slotted laser design is presented in Section V. It is shown that fast wavelength switching is possible between a significant number of wavelength channels in these relatively simple devices that also achieve excellent SMSR and linewidth over their tuning range. II. THEORY A. Transmission Matrix and Inverse Problem Solution It has long been also known that scattering centers along the length of an FP laser can strongly perturb the lasing spectrum of the device [16], [17]. In many cases, single-mode emission can result. This observation was built upon and taken further when single-mode operation in FP lasers was achieved by intentionally etching slots into the laser ridge waveguide [14], [18]. If we imagine the slots as scattering centers distributed along the FP laser cavity, the problem of relating the index profile in real space to the threshold gain modulation in wavenumber space is naturally expressed as an inverse problem. Consider a 1-D model of the FP cavity geometry as shown in Fig. 1. The system comprises an FP cavity of length Lc , the mirror reflectivities are r1 and r2 , and N additional index steps are introduced. A detailed analysis of the characteristics of the index steps is given in [18]. By treating the effect of these additional index step features on the lasing mode structure of the cavity as a perturbation, a set of self-consistent equations for the lasing mode frequencies and their thresholds can be derived [20]. Suppose the transfer matrix T relates the right and left moving electric fields at the cavity mirrors. Then, the lasing modes of the cavity are defined by the relation T11 + r1 r2 T22 = r1 T21 − r2 T12 .

(1)

For the plain FP laser, with Δn = n1 − n2 = 0, the diagonal elements of T are T11 = exp(−iΣθi ) and T22 = exp(iΣθi ), where θi is the complex optical path lenght across each

´ O’BRIEN et al.: DESIGN, CHARACTERIZATION, AND APPLICATIONS OF INDEX-PATTERNED FABRY–PEROT LASERS

section of the leser. In this case, the active region is homogeneous in the longitudinal direction and the off-diagonal elements of T are T21 = T12 = 0. The lasing condition is then given by 1−r1 r2 exp(2iΣθi ) = 0. From the real and imaginary parts of the lasing condition, we find the threshold gain and cavity resonance condition, respectively. Assuming the gain is uniformly distributed along the device, we derive the familiar expression for the threshold gain of the plain FP cavity 1 1 FP ln . (2) Gth = Lc r1 r2 For the case where Δn = 0, the lasing condition at first order in Δn/n can be shown to be [19] θi 1 − r1 r2 exp 2i =i

+

Δn . (3) sin θ2j r1 exp 2iφ− j + r2 exp 2iφj n j

This equation describes the coupling between each feature and the external mirrors exactly, but neglects any coupling between the features themselves θj- , θj+ are the complex optical path length from the center of the jth feature to the left (−) and right (+) mirrors, respectively. To obtain the lasing mode frequencies and their thresholds in the perturbed case, (3) is expanded around the cavity resonance condition allowing for a shift in the lasing mode frequency from the unperturbed value. The inverse problem at first order can then be solved by choosing a particular cavity resonance m0 as an origin in wavenumber space. We assume quarter wave features in order that the intensity scattered by each feature at the wavelength of mode m0 is maximized. Taking the limit of a vanishing index step, we at first neglect the optical path length corrections that result from the introduction of the additional features. In this limit, distances along the cavity are proportional to the corresponding change in the optical path. One finds that the effect of the spatially varying refractive index is maximized where each feature is placed such that a half-wavelength subcavity at the wavelength of mode m0 is formed between the feature and one of the external mirrors. If we then label each resonance of the cavity m, where m = m0 + Δm, the threshold gain function takes a particularly simple form Gth = GFP th + ×

N

Δn cos(m0 π) cos(Δmπ) √ nLc r1 r2

A(εj ) sin(2πεj m0 ) cos(2πεj Δm).

(4)

j =1

In the aforementioned expression − |r2 | exp −εj Lc GFP (5) A (εj ) = |r1 | exp εj Lc GFP th th where εj is the fractional position of the center of each feature measured from the center of the cavity. The change in threshold gain is now expressed with respect to Δm and Fourier analysis can be used in order to build up a particular threshold gain modulation in wavenumber space. We take the product of the Fourier transform of the desired threshold

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modulation with the envelope function [A(ε)]−1 . The absolute value of this product determines the feature density function, which is sampled over the appropriate interval in order to approximately reproduce the desired threshold gain modulation. The approximate feature positions are given by the solutions of the following equation: εj 1 (6) [A(x)]−1 |F (x)|dx = j − . C 2 Here, C is normalized to the number of features to be introduced, F is the Fourier transform of the desired threshold gain modulation function, and j = 1, 2, . . . , N . The positions of the features are then adjusted so that half-wave and quarter-wave subcavities are formed to the left and right as appropriate. Corrections for the finite index step associated with each feature are accounted for at this final stage. B. Design of a Single-Mode Laser Using Inverse Technique Where single-mode emission is required, the threshold gain spectrum would ideally show a reduction in threshold gain for the selected mode m0 while modes with m = m0 would remain unperturbed. A function with this property is sinc(Δm) = sin(πΔm)/(πΔm). This function can be written as the Fourier transform of the unit rectangle, Π(ε), centered at ε = 0. That is, ε=1/2 cos (2πε Δm) dε. (7) sinc (Δm) = ε=−1/2

To make a connection between the index profile in this case and grating-based approaches, assume first that the end mirror reflectivities are equal. The envelope function [A(ε)]−1 diverges and changes sign at the device center in this case. Therefore, when adjusting the feature positions for resonance, a π/2 phase shift or equivalently a half-wave subcavity is found at the device center. This is precisely the distribution of subcavities found in a phase-shifted DFB laser. However, in our case the feature density is nonuniform and must diverge at the device center. To avoid the problem of this divergence, an asymmetric cavity with one high-reflection (HR) mirror is advantageous. This allows features to be placed on the opposite side to the HR mirror where the feature density is more uniform. In any real device, the cavity resonances of interest are contained within a finite bandwidth determined by the gain medium. We can, therefore, refine our desired threshold gain modulation by defining a periodic distribution of sinc functions with spacing a fundamental cavity modes. This comb of modes is then multiplied by a Gaussian envelope function, giving the ideal threshold modulation ∞ 2 2 ∼ exp[−πτ (Δm) ] sinc(Δm − na) (8) Gth − GFP th n =−∞

where τ determines the width of the Gaussian. The Fourier transform of this ideal threshold modulation is a series of Gaussian functions with spacing a−1 within the window − 21 < ε < 12 . Fig. 2(a) shows the feature density function for a single-mode laser with one HR mirror obtained in this way. In this example, a = 20 and τ was chosen such that the satellite modes at

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Fig. 3. (a) Optical spectra showing single-mode operation at three different temperatures: −5, 25, and 90 ◦ C. (b) Temperature dependence of wavelength for an index-patterned single-mode laser. Fig. 2. (a) Feature density function. Inset: the calculated threshold gain spectrum. Lower panel: cavity geometry. The device is HR coated as indicated. (b) Emission spectrum for the single-mode laser and an FP laser (inset) with a drive current at twice threshold. The single-mode laser has an SMSR in excess of 50 dB.

Δm = ±20 experience an 80% reduction in the change in threshold gain compared to the value at m0 . The free space wavelength of mode m0 was chosen to be 1547.5 nm with a cavity length of 300 μm (m0 = 1236). The device schematic that results with N = 19 is shown in the lower left panel of Fig. 2 and the calculated threshold gain of modes is shown in the inset. Despite the small number of features introduced, the calculated threshold gain modulation is in good agreement with the ideal form. The optical spectrum at twice threshold of the fabricated device is shown in Fig. 2(b). One can see that a single primary mode is selected at a predetermined wavelength. The SMSR at twice threshold also exceeds 50 dB. For comparison, the optical spectrum at twice threshold of a plain FP laser fabricated on the same bar is shown in the inset. III. CHARACTERIZATION OF SINGLE-MODE INDEX-PATTERNED LASERS Applications in the access and enterprise markets require devices to operate without a thermoelectric cooler in a predictable fashion over a specified temperature range depending on the precise application. The typical temperature ranges are −5 ◦ C ≤ T ≤ 85 ◦ C for wavelengths around 1550 nm and 1490 nm, and −40 ◦ C ≤ T ≤ 85 ◦ C for wavelengths around 1310 nm, with 5-mW emission required at the upper temperature. In addition, single longitudinal mode operation, with an SMSR of at least 40 dB, is required over these specified temperature ranges. A class of index-patterned lasers used for single-mode wavelength output is known as discrete mode (DM) lasers [1]. Wide temperature range measurements of one such DM laser were carried out using a Thermonics T-2420 precision temperature forcing system, which allowed the device to be operated in a thermal environment ranging from −50 ◦ C ≤ T ≤ 100 ◦ C. While index patterning approaches to the design of single-frequency lasers can be adapted to any of the three key transmission windows: 1550, 1310, and 1490 nm, we will show spectral results here for 1550-nm operation.

Optical spectra of a single-mode DM laser for temperatures ranging between −5 ◦ C ≤ T ≤ 90 ◦ C are shown in Fig. 3(a). The measured threshold currents Ith were 9, 12, and 37 mA at temperatures −5, 25, and 90 ◦ C, respectively. At a fixed current of 60 mA, the output power at these operating temperatures was 14, 13, and 5 mW, respectively [1]. Fig. 3(b) shows the plot of the emission wavelength as a function of temperature. The lasing wavelength shows a linear dependence with temperature with a tuning rate Δλ/ΔT ≈ 0.1 nm/◦ C, which is consistent with the expected mode shift due to the temperature-induced change in the refractive index. We note that this wavelength change with temperature is characteristic of the temperature dependence of the refractive index of the materials used and is the same as for DFB lasers made from the same materials. An SMSR > 40 dB is achieved across the full temperature range. In addition to large SMSR, another important characteristic of single-frequency semiconductor lasers is the lasing mode linewidth. Narrow linewidth emission is critical if applications in optical coherent communications, sensing and optical clocks are to be realized. Ideally, this property would be achieved in a monolithic device, with the least associated level of device complexity and low component cost. While narrow linewidth monolithic devices have been reported in certain optimized devices [21], the free-running linewidth of typical DFB and DBR laser devices is in the 2–20 MHz range [22], which restricts their use in such applications. An important feature of the DM laser is that the multiple quantum well active region is fabricated in the InGaAlAs material that has a high differential gain resulting in a low linewidth enhancement (alpha) factor of approximately 2–3 as opposed to approximately 5 for an InGaAsP-based DFB [23]. Here, we show linewidth measurements obtained from a monolithic 1550-nm laser design with a cavity length of 1100 μm, packaged in a hermetically sealed 14-pin butterfly package containing a thermoelectric cooler, thermistor, and optical isolator. The linewidth was measured using the delayed self-heterodyne (DS-H) measurement technique as described in [24]. In Fig. 4, we show the measured frequency resolved line of the laser measured at a bias current of 200 mA and a chip temperature of 25 ◦ C. The spectrum was fitted with a Lorentzian lineshape and yields a linewidth of ∼200 kHz. The inset of Fig. 4 shows a plot of the emission wavelength and linewidth as a function of temperature. As was the case in Fig. 3(b), the lasing

´ O’BRIEN et al.: DESIGN, CHARACTERIZATION, AND APPLICATIONS OF INDEX-PATTERNED FABRY–PEROT LASERS

Fig. 4. Measured lineshape of a single-mode 1550 nm DM laser with a bias current of 200 mA at 25 ◦ C. Inset: temperature dependence of linewidth and wavelength.

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Fig. 6. (a) Feature density function. The dashed lines are the negative of the feature density function in intervals where the Fourier transform of the spectral filtering function chosen is negative. Inset: calculation of the threshold gain of modes for the laser cavity schematically pictured in the lower panel of the figure. Lower panel: laser cavity schematic indicating the locations of the additional index step features. The device is HR coated as indicated. (b) Optical spectrum of the device at a device current of 43.5 mA (two-color point). Inset: background-free intensity autocorrelation measurement showing mode beating at 480 GHz.

IV. DUAL AND MULTIMODE INDEX-PATTERNED LASERS

Fig. 5. Modulation response of a 1310-nm DM laser showing a modulation bandwidth >10 GHz over a range of different drive currents.

wavelength shows a linear dependence with temperature with a tuning rate Δλ/ΔT of 0.1 nm/◦ C. Significantly, the linewidth remains relatively stable with a value 30 dB, which can be accessed via current tuning alone. The upper portion of Fig. 8(a) shows the channel at 1556.55 nm while Fig. 8(b) shows 25 of these single-mode spectra overlaid, showing a complete tuning range of almost 20 nm. As we discussed in the context of single-mode semiconductor devices, narrow linewidth emission becomes increasingly important as optical networks begin to employ advanced optical modulation formats and digital coherent receivers. The linewidth of the three-section index-patterned laser described previously was measured (again using the DS-H technique) for each of the 25 100-GHz spaced channels. The spectrum of the 1556.3 nm channel is shown in Fig. 9(a), and the linewidth was measured to be 460 kHz, while the linewidth of each of the channels, together with the corresponding SMSR, is

Fig. 9. (a) Lineshape of the index-patterned tunable laser tuned to 1556.3 nm. (b) Linewidth and SMSR at each of 25 adjacent 100-GHz spaced ITU channels. The dashed lines show the average values.

displayed in Fig. 9(b). The average linewidth was 538 kHz, while the maximum linewidth for the measured channels was 738 kHz. The average SMSR was >35 dB. In agreement with the Shawlow–Townes–Henry relationship between linewidth and optical power, the channel with highest measured linewidth also exhibited the lowest power. In addition to its low-phase noise, characterized by narrow linewidth, the device also exhibits low relative intensity noise with a worst case for the channels measured of −135 dB/Hz. Further details regarding the static characteristics of these devices can be found in [14], [45], and [46]. In today’s networks, where tunable lasers are mainly employed for inventory reduction purposes, switching times in the millisecond or even second timescales generally suffice. However, faster switching is desirable for a number of reasons. In particular, faster switching times enable or enhance the more advanced networking functions including dynamic bandwidth provisioning, protection and restoration, and optical switching. The switching time of the index-patterned tunable laser illustrated in Fig. 8 was measured using a DS-H technique [47] while the emission wavelength was switched by applying a square wave with a rise time