Simple detuning method for low-chirp operation in polymer-based tunable external-cavity lasers

We propose and demonstrate a simple detuning method for the low-chirp operation of a polymer-based tunable external-cavity laser (ECL). To ensure the low-chirp operation of this directly-modulated ECL, we first obtain the optimum values of the heater current applied to the polymer Bragg grating reflector (PBR) and the operating temperature of this ECL. For this purpose, we sweep the current applied to the phase control heater until the peak output power measured from the high-reflection (HR) coated facet reaches the minimum value. We then operate this ECL with minimum chirp by tuning the lasing mode to the longer wavelength limit of the stable operation region. This is because the detuned loading effect is maximized at this limit as the in-phase condition between the lights reflected from the PBR and anti-reflection (AR) coated facet of the gain medium is satisfied. Thus, by using this method together with conventional wavelength-locking algorithm, we can operate this ECL with minimum chirp at any wavelength. ©2015 Optical Society of America OCIS codes: (130.5460) Polymer waveguides; (140.3600) Lasers, tunable; (230.1480) Bragg reflectors; (130.3120) Integrated optics devices; (250.5960) Semiconductor lasers. References and links 1. B.-S. Choi, S. H. Oh, K. S. Kim, K.-H. Yoon, H. S. Kim, M.-R. Park, J. S. Jeong, O.-K. Kwon, J.-K. Seo, H.-K. Lee, and Y. C. Chung, “10-Gb/s direct modulation of polymer-based tunable external cavity lasers,” Opt. Express 20(18), 20368–20375 (2012). 2. K. Vahala and A. Yariv, “Detuned loading in coupled cavity semiconductor lasers-Effect on quantum noise and dynamics,” Appl. Phys. Lett. 45(5), 501–503 (1984). 3. G. P. Agrawal and C. H. Henry, “Modulation performance of a semiconductor laser coupled to an external highQ resonator,” IEEE J. Quantum Electron. 24(2), 134–142 (1988). 4. B.-S. Choi, J. S. Jeong, K.-H. Yoon, K. S. Kim, H. S. Kim, M.-R. Park, O.-K. Kwon, H.-K. Lee, and Y. C. Chung, “Evaluation of chirp reduction in polymer-based tunable external-cavity lasers,” IEEE J. Quantum Electron. 51, 2000315 (2015). 5. B. Tromborg, H. Olesen, X. Pan, and S. Saito, “Transmission line description of optical feedback and injection locking for Fabry-Perot and DFB lasers,” IEEE J. Quantum Electron. 23(11), 1875–1889 (1987). 6. M. F. Ferreira, J. F. Rocha, and J. L. Pinto, “Noise and modulation performance of Fabry-Perot and DFB semiconductor lasers with arbitrary external optical feedback,” IEE Proc., J Optoelectron. 137(6), 361–369 (1990). 7. E. Detoma, B. Tromborg, and I. Montrosset, “The complex way to laser diode spectra: example of an external cavity laser strong optical feedback,” IEEE J. Quantum Electron. 41(2), 171–182 (2005). 8. H. Klein, “Hybrid InP-Polymer 30 nm tunable DBR Laser for 10 Gbit/s direct Modulation in the C-Band,” in Proceedings of International Conference on Indium Phosphide and Related Materials (IEEE, 2012), 20–21. 9. S. K. Kim and J. Jeong, “Transmission performance on frequency response of receivers and chirping shape of transmitters for 10 Gb/s LiNbO3 modulator based lightwave systems,” Opt. Commun. 175(1-3), 109–123 (2000). 10. A. Uskov, J. Mørk, and J. Mark, “Wave mixing in semiconductor laser amplifiers due to carrier heating and spectral-hole burning,” IEEE J. Quantum Electron. 30(8), 1769–1781 (1994). 11. H. Kuwatsuka, T. Simoyama, and H. Ishikawa, “Enhancement of third-order nonlinear optical susceptibilities in compressively strained quantum wells under the population inversion condition,” IEEE J. Quantum Electron. 35(12), 1817–1825 (1999). #251414 Received 5 Oct 2015; revised 11 Nov 2015; accepted 11 Nov 2015; published 16 Nov 2015 © 2015 OSA 30 Nov 2015 | Vol. 23, No. 24 | DOI:10.1364/OE.23.030657 | OPTICS EXPRESS 30657 12. N. Ogasawara, R. Ito, and R. Ito, “Longitudinal mode competition and asymmetric gain saturation in semiconductor injection lasers: II. Theory,” Jpn. J. Appl. Phys. 27(1), 615–626 (1988). 13. A. Godard, G. Pauliat, G. Roosen, and E. Ducloux, “Modal competition via four-wave mixing in single-mode extended-cavity semiconductor lasers,” IEEE J. Quantum Electron. 40(8), 970–981 (2004).


Introduction
A polymer-based tunable external-cavity laser (ECL) capable of operating at 10 Gb/s has been recently developed as a low-cost light source for next-generation passive optical networks [1].This ECL is implemented simply by butt-coupling a polymer-based tunable reflector with a high thermo-optic coefficient (~-2.5x10−4 /°C) to a high-speed superluminescent diode (SLD).It has been reported that the transmission distance achievable by this ECL can be considerably extended by using the detuned loading effect owing to the large stable operation region toward the longer wavelength from the peak of the Bragg reflection [1][2][3][4].For example, by using this effect, we have recently demonstrated the transmission of a 10-Gb/s signal over 20 km of standard single-mode fiber (SSMF) with a power penalty of <2 dB [4].However, this performance can be obtained only when we detune the lasing mode of the ECL to the longer wavelength limit.Thus, for the practicality, it is necessary to develop an efficient method for operating the ECL under this detuning condition.
The detuning of the lasing mode can be achieved by shifting the comb-mode spectrum of the ECL against the peak wavelength of the Bragg reflection.Thus, for this purpose, we need to adjust both the grating heater (which controls the peak wavelength of the reflector) and the phase control (PC) heater (which fine-tunes the comb mode) simultaneously.Several measurement methods can be used for such adjustments.For example, we can trace the movement of the lasing mode to the longer wavelength region by measuring the optical spectrum of the ECL.We can also identify the low-chirp operating condition of the ECL by measuring the eye diagram or bit-error rate (BER) of a high-speed signal after a transmission over a sufficient length of SSMF.However, these methods are not useful when a parasitic reflection occurs at the anti-reflection (AR) coated facet of the SLD.This is because the amount of chirp is highly dependent on this parasitic reflection [4].Thus, to operate the ECL with minimum chirp at any wavelength, it is necessary to utilize another parameter that controls the variation caused by the parasitic reflection.However, owing to this additional parameter, we cannot identify the minimum chirp by using the method based on the optical spectrum.A method based on a BER measurement also becomes inefficient owing to the substantially increased measurement time caused by the use of three different parameters.In fact, to the best of our knowledge, there has been no report yet on an efficient method for identifying the low-chirp operating condition of the ECL achievable by using the detuned loading effect.
In this paper, we propose a simple detuning method for the minimum-chirp operation by monitoring the output power from the high-reflection (HR) coated facet (referred to as 'HR power' in this paper).We choose this power rather than the output power from the polymer Bragg grating reflector (referred to as 'PBR power') because the power variation is much larger at the HR-coated facet.In addition, this HR power has a similar trend to the wavelength shift of the lasing mode in the longer wavelength region while sweeping the current applied to the PC heater.For this method, we first measure the output power while sweeping the current applied to the PC heater under various conditions.We then identify the optimal values of the grating heater current and the operating temperature from where the minimum peak power is observed in this swept curve.After setting these two values, we can achieve the lowest chirp by detuning the lasing mode to the longer wavelength limit with the PC heater.This is because the in-phase condition between the lights reflected from the PBR and the AR-coated facet is satisfied at the longer wavelength limit while sweeping this PC current [4].On the other hand, these lights become out-of-phase near the zero detuning, where the HR power is maximized [4].To verify the effectiveness of this method, we directly modulate the fabricated ECL at 10 Gb/s with an extinction ratio of ~6 dB and evaluate its performance after the transmission over 20-km-long standard single-mode fiber (SSMF).The results show that we can indeed obtain the minimum power penalty under the conditions set by the proposed detuning method.We also confirm that this method can be integrated with a wavelengthlocking algorithm to ensure the minimum-chirp operation at any available wavelength.

Operating principles
To develop a method for setting the optimal chirp conditions of a tunable ECL, it is crucial to understand the internal state of the device under such conditions and then choose a suitable monitored parameter that best reflects the chirp variations.For this purpose, we investigate the relation between the device parameters and the controllable variables for the proposed ECL structure.Based on these results, we estimate the changes in the effective line-width enhancement factor (LEF) as we detune the lasing mode, particularly for the case of a parasitic reflection.Finally, we assess the suitability of the output power as a monitored signal of the chirp conditions by analyzing how the output power is determined.The ECL model utilized for the analysis is based on a transmission line method, and the model parameters used are listed in Table 1 [4][5][6][7].

Device structure and control parameters of the tunable ECL
Figure 1 shows a schematic diagram of the polymer-based tunable ECL analyzed in the present paper.Some of the special features designed for use in high-speed, long-distance transmission systems are as follows.We use a high-speed SLD as the gain medium.This SLD has a lateral taper structure to enhance the coupling efficiency to the PBR [8].The PBR is composed of a grating section, which acts as an external reflector with a wide tuning range, and a PC section, which tunes the position of the comb modes of the ECL.A grating with a low coupling coefficient (i.e., κ = 150 m −1 ) is engraved on the waveguide of the PBR.For high-speed operation, we minimize the total cavity length (round-trip time of ~30 ps) of the ECL by butt-coupling the SLD to the PBR [1,4].As a result, the ratio of the ECL mode spacing (~0.24 nm) to the PBR spectral bandwidth (which can be used as a figure of merit for characterizing the achievable detuning) is designed to be ~1.
The ECL parameters related to the mode detuning are the peak wavelength of the external reflector (denoted as λ PBR ) and the phase angle of the comb modes.However, when a reflection occurs at the AR-coated facet, we should consider two phase angles, one (denoted as φ 2 ) of which is solely related to the phase condition of the ECL modes, and the other (denoted as φ 1 ) is related to both the phase condition of the ECL modes and the spectral reflectivity seen by the gain medium (, i.e., "right reflectivity," denoted as r R ) [4].The dependence of the spectral reflectivity on the phase angle is related to the formation of an etalon-like cavity mirror placed between the AR-coated facet and the PBR.The light reflected from the PBR interferes with the parasitically reflected light at the AR-coated facet, and as a result, the spectral shape of r R is distorted from that of the ideal grating reflectivity (blue curve) to that of the effective reflectivity (red curve), as shown in Fig. 2(a).Thus, when φ 1 is monotonically adjusted, the reflectivity peak shows a cyclic movement between the higher and lower detuned frequencies [4].The importance of this reflectivity change is that the achievable chirp by this tunable ECL is significantly influenced by φ 1 .Figures 2(b) through 2(d) show the r R for φ 1 of 0°, 120°, and 180°, respectively.The corresponding LEFs achieved by the detuned loading effect are represented through the blue curves [4].The blue dots on the curves denote the positions of the longer wavelength limits where the minimum LEFs for the given phase angles are obtained.These positions of the stability limits are calculated by applying a linear stability analysis to the model based on a transmission line description [4,7].It is notable that the global minimum LEF is obtained when φ 1 = 0° (i.e., under the inphase condition).In the case of φ 1 = 120°, where the peak of r R moves to the higher frequency, the reduction of the LEF (from 5 to 2.22) is not sufficient for improving the longdistance transmission performance [9].When the out-of-phase condition (φ 1 = 180°) is met, the peak reflectivity is lowest, and the minimum LEF is located between the two former LEFs.Thus, owing to this drastic change in the minimum achievable LEF, a separate control of each phase angle is indispensable.
Because the control of three different parameters (φ 1 , φ 2 , and λ PBR ) is required to obtain the global minimum LEF, we should use three input variables to change these parameters.The current injected to the PC heater, I PC , changes only the refractive index of the PC section through a thermo-optic effect.Thus, it changes only φ 1 .The grating heater current, I grating , is basically used for adjusting the peak wavelength of the Bragg reflection.However, about half of the grating section is included in the laser cavity, and thus, the change in the refractive index also contributes to a varying φ 1 .The phase angle in the gain medium, φ 2 , can be controlled only by changing the operating temperature of the ECL module, T module , through a thermo-electric cooler (TEC) placed below the entire butt-coupled module.Because T module affects all sections within the laser cavity, φ 1 and λ PBR are also influenced by it.It should be noted that owing to these entangled dependencies of the ECL parameters, finding a detuning method for a low chirp operation is not a simple task.

Output power
Because the output power is a promising candidate for a monitored signal of detuning, we estimated the two powers emitted from each facet of the ECL numerically.Figure 3(a) shows how the output powers are determined in the ECL when φ 1 is swept at φ 2 = 105°.Here, a φ 1 sweep moves the comb modes of the ECL and corresponds to the PC current sweep during the experiment.The red curve is the ratio of the PBR power to the AR power (i.e., power from the AR-coated facet).The AR power is gradually attenuated through the imperfect coupling between the SLD and the PBR, the waveguide loss in the PBR, and the reflection of the Bragg grating as it passes through the PBR.Owing to the spectral shape of r R , this ratio has the minimum value near the zero detuning.Contrary to this dependence, the AR power has its maximum value near the zero detuning because a higher feedback by r R induces higher AR power.The kink shown in AR power is due to the abrupt change in r R near the detuned frequency of −2.4 GHz (i.e., from the flattened shape of Fig. 2(d) to the symmetric one of Fig. 2(c) with respect to zero detuned frequency).Because the PBR power is a product of these two factors, the resulting frequency dependence of this power is very small, as shown in the red line of Fig. 3(b).It is favorable to have a relatively small change of power in case it is used as the output power of the ECL, although as a monitored signal for detuning, this is inappropriate.In contrast, the HR power (blue line) experiences a drastic power change because two factors (i.e., AR power and r R ) determining this power have a similar spectral dependence.In addition, the blue line in Fig. 3(b) shows that the HR power monotonically decreases as the lasing mode is detuned toward the longer wavelength region.Thus, it is better to choose the HR power rather than the PBR power as a monitored signal owing to its larger and monotonic power variation during detuning.

Experiment
To develop a detuning method for a low-chirp operation experimentally, we fabricated a polymer-based tunable ECL and evaluated its detuning characteristics.We first compared the variations of the PBR and HR powers with the wavelength shift of the lasing mode to confirm their suitability as a monitored signal.We then measured the peak HR power gathered during the PC current sweep, and the power penalty after the transmission over the 20-km long SSMF as functions of the wavelength and operating temperature.Finally, we utilized this method to obtain the trajectory of the minimum-chirp condition that can be used to control the wavelength of the tunable ECL.

Fabrication
A high-speed SLD having a tapered waveguide structure was utilized in the ECL [8].The gain medium supports a large modulation bandwidth of over 10 Gb/s, and the laterally tapered waveguide reduces the far field angle of the SLD in the horizontal and vertical directions to < 20°.To reduce the facet reflection to a negligible level, AR-coating was applied to the 9°-tilted facet and a high-reflection coating was applied to the other facet.
The PBR was implemented by engraving a 3.5-mm long fifth-order Bragg grating on a 5μm x 5-μm waveguide core [1].The resulting reflectivity and spectral bandwidth were 21% and 0.24 nm, respectively.A metal heater was deposited on the upper cladding layer of the grating section to change the refractive index through the thermo-optic effect.To realize the PC section, another 0.5-mm long heater was deposited on the waveguide portion without a grating.The PBR also had tilted facets on both sides to minimize the reflection.The output of the ECL was coupled through a pigtailed fiber, and the power through the HR-coated facet was measured by placing the monitor photodiode at the backside of the SLD.
The SLD and PBR were butt-coupled to minimize the round-trip time of the photon for high-speed operation.Figure 4 shows a photograph of the UV-cured tunable ECL module.The operating temperature of this module was monitored using a thermistor mounted on a silicon plate and controlled through a thermo-electric cooler placed below the entire module.
The tunable ECL has a threshold current of between 6 and 10 mA depending on the degree of detuning, and the output power is ~5 mW at 50 mA.To obtain a tuning range of 16 nm (from 1547 to 1531 nm), a grating heater power of about 85 mW is needed.

Two output powers of tunable ECL
To choose a more appropriate monitored signal experimentally, we measured the PBR and HR powers during the current sweep on the phase control heater.We obtained another sweep curve by changing the sweep direction to check the existence of a hysteresis region [4].In this measurement, part of the PBR power was used for a spectrum analysis, and the HR power was monitored by a photodiode placed near the HR-coated facet.To take into account the effect of the large signal modulation on the stable operation region, we also applied a 9.953-Gb/s modulation signal with a modulation depth, V pp , of 3.4 V at a bias current of 70 mA [4].The extinction ratio was measured to be ~6 dB under these conditions.The results in Fig. 5 show that the hysteresis region, generated by a nonlinear gain, nearly disappears owing to the chirp-induced mode hopping at the boundary of the stable operation region [4,7,[10][11][12][13].Thus, we can obtain only one detuned state for the given PC current under this large modulation depth.
As estimated in section 2.2, the variation of the PBR power during detuning was very small (<5%) and at a longer wavelength region, the power was not as monotonically changed as the wavelength.On the contrary, the photodiode current (which is proportional to HR power) shows about a 67% variation and a large power jump is induced when the modehopping occurs.As a result, we choose the HR power as a monitored signal for detuning.

PC current sweep
Because the PC current is the only input variable controlling the phase angle (φ 1 ) without affecting the peak wavelength of the PBR, we utilized this current to achieve a longer wavelength detuning.It should be noted that we can obtain the global minimum-chirp condition (a longer wavelength limit when φ 1 = 0°, as shown in Fig. 2(b)) by appreciating its characteristic behavior in power.Figure 6(a) shows typical PC current sweep curves measured at different grating heater currents.The shapes of the power variations are similar to each other, but the peak powers are different.The reason for this behavior is that the starting phase angles of φ 1 are different for each curve owing to the change in the peak wavelength of the PBR, and thus the peak power, determined at the peak reflectivity during the detuning, and consequently located near the zero detuning, is changed.Accordingly, the peak position of the magenta line in Fig. 6 We show one axis of this figure using a more significant lasing wavelength instead of the grating heater current.We can clearly see the cyclic behavior of the peak photodiode current to the directions of the wavelength and temperature axes.

Transmission performances
We measured the transmission performances to find a method for obtaining the minimumchirp conditions from the PC current sweep curves.The tunable ECL was directly modulated at 9.953 Gb/s with a pattern length of 2 31 -1, and its bit-error ratio (BER) curves were measured over 20-km long SSMF.We set the bias and modulation currents of the ECL to 50 mA and 59 mA p-p , respectively.The extinction ratio was measured as ~6 dB under these conditions, and an APDFET receiver with a sensitivity of −23.5 dBm (@ BER of 10 −12 ) was used for the signal detection.It should be noted that the shapes of these two curves are very similar to each other, which is due to about a 180° change in φ 1 when the lasing mode is detuned from the peak HR power position to a longer wavelength limit.For example, in the case of the minimum power penalty, the optimum in-phase condition (shown in Fig. 2(b)) between the lights reflected from the PBR and the AR-coated facet is met at the longer wavelength limit, despite these lights becoming out-of-phase (shown in Fig. 2(d)) near the zero detuning where the maximum HR power is obtained.Thus, we can obtain the optimal values of the grating heater current and the operating temperature from the conditions of the PC current sweep curve with the the minimum-chirp trajectory by controlling the operating temperature and grating heater current.Here, the PC heater current is adjusted to the value where the photodiode current is same.The next trajectory can be used in case the change in the operating temperature is too much.Fig. 8. Peak HR powers (contour plot) and some power penalties (at circled conditions) obtained after a transmission over 20-km long SSMF to demonstrate the wavelength and operating temperature dependences of the minimum-chirp conditions.

Summary
We developed a simple detuning method for the minimum-chirp operation of a tunable ECL, capable of being directly modulated at 10 Gb/s.The proposed method starts by choosing the optimal conditions of the grating heater current and the operating temperature.For this purpose, we measured the variations in HR power while sweeping the current applied to the PC heater for various combinations of these two variables.Under the optimized conditions, the ECL showed the lowest peak HR power because the feedback light from the PBR was out-of-phase with the light reflected parasitically from the AR-coated facet of the gain medium, whereas these lights were in-phase when the lasing mode reached the longer wavelength limit.This was due to the change in the phase difference between these lights occurred during the sweep.We finally achieved the minimum-chirp operation by detuning the lasing mode to the longer wavelength limit of the stable operation region, where the HR power was about 20% larger than the value measured at the minimum power position.To check the possibility of integrating a wavelength-locking algorithm with this detuning method, we measured the wavelength dependence of the minimum-chirp conditions.The results indicated that we could operate the ECL under the minimum-chirp conditions at all available wavelengths owing to the periodic appearance of this condition with a wavelength dependence of ~6.5°C/nm.The long-term stability of this ECL can be ensured by using the wavelength-locking technique integrated with the proposed detuning method.Thus, by using the proposed method, we could utilize a polymer-based tunable ECL as a low-cost light source for providing >10-Gb/s service to each subscriber in the next-generation passive optical network.

Fig. 1 .
Fig. 1.Schematic diagram of the polymer-based tunable ECL module and the device parameters and control variables for detuning the lasing mode.

Fig. 2 .
Fig. 2. (a) Simulated grating reflectivity (blue line), right reflectivity r R (red line), and reflectivity of the cavity mirror (green line, not to scale) of the polymer-based tunable ECL as a function of the detuned frequency, and r R , effective LEF, and minimum achievable LEF (blue dot) at φ 1 of (b) 0°, (c) 120°, and (d) 180°.

Fig. 3 .
Fig.3.Simulated results for (a) the ratio of power through the PBR to the power through the AR-coated facet and the absolute value of the latter, and (b) the resulting PBR and HR power as a function of the detuned frequency (@ φ 2 = 105°).

Fig. 5 .
Fig. 5. Comparison of the measured lasing wavelength of the tunable ECL with power through the (a) PBR and (b) HR-coated facet when a 9.953-Gb/s modulation signal with a modulation depth of V pp = 3.4V is applied at a bias current of 70 mA.
(a) corresponds to φ 1 = 0°.A more comprehensive plot is drawn in Fig. 6(b) where only the peak values of the photodiode currents during a PC current sweep are shown as functions of the wavelength and operating temperature, T module .

Fig. 6 .
Fig. 6.(a) Typical HR power curves while sweeping the phase control current (shown in applied power) with ten different grating heater currents, I grating , and (b) a surface plot drawn using the peak values of the HR power during a phase control current sweep as functions of the wavelength (converted from the grating heater current) and operating temperature, T module .

Figure 7 (
a) shows longer wavelength detuned power penalties for various I grating values with the temperature fixed at 25°C.The corresponding peak values of the photodiode current during each PC current sweep are also shown in the figure.The power penalties were measured at a BER of 10 −4 , over which an error floor was not observed.