Ultra-narrow linewidth quantum dot coherent comb lasers with self-injection feedback locking

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Optics Express, 26, 9, pp. 11909-11914, 2018

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Ultra-narrow linewidth quantum dot coherent comb lasers with

self-injection feedback locking

Lu, Z. G.; Liu, J. R.; Poole, P. J.; Song, C. Y.; Chang, S. D.

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Ultra-narrow linewidth quantum dot coherent

comb lasers with self-injection feedback

locking

Z. G. L

,

_{J. R. L}

_{, P. J. P}

_{, C. Y. S}

_,

_{S. D. C}

*Zhenguo.Lu@NRC-CNRC.GC.CA

Abstract: _{We have used an external cavity self-injection feedback locking (SIFL) system to} simultaneously reduce the optical linewidth of over 39 individual wavelength channels of an InAs/InP quantum dot (QD) coherent comb laser (CCL). Linewidth reduction from a few MHz to less than 200 kHz is observed. Measured phase noise spectra clearly indicate a significant decrease in phase noise in the frequency range above 2 kHz. The RF beating signal between two adjacent channels also shows a substantial reduction in 3-dB linewidth from 10 kHz to 300 Hz with the SIFL system, and a corresponding drop in baseline level (−27 dB to −50 dB).

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

OCIS codes: _{(060.4510) Optical communications; (230.5590) Quantum-well, -wire and –dot devices; (140.5960)}

Semiconductor lasers; (140.4050) Mode-locked lasers; (140.3520) Lasers, injection-locked; (300.3700) Linewidth.

References and links

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1. Introduction

The enormous growth of internet data traffic imposes new challenges on the telecommunication industry, requiring the development of new advanced photonics components for next generation optical networks [1]. Recently, there has been significant interest in optical comb lasers and their benefits as monolithic sources of multiple wavelength channels for wavelength division multiplexing (WDM), dense-WDM (DWDM), super-channel, and coherent fiber communication systems with net data rates exceeding Terabit/s transmission rates and high spectral efficiency [2, 3]. Such comb lasers can reduce costs and packaging problems by replacing many separate lasers for each channel by a single laser chip. In particular semiconductor-based monolithic coherent comb lasers (CCLs) are additionally very attractive laser sources owing to their compactness, low cost, large mode spacing, low noise and high power performance [4–6]. Narrow optical linewidth is an essential requirement for CCLs to be used in high data rate coherent communication systems where phase noise impacts signal noise in the coherent detection process, and the maximum data rate is determined by the ratio of signal power to noise power [7]. The phase noise for a laser line is related to its optical linewidth, so linewidth is often used as a measure of its suitability for use in coherent systems. For example a terabit coherent transmission system requires linewidths of a few hundred kHz or less [8]. Recently we have demonstrated InAs/InP quantum-dot (QD) mode-locked lasers (MLLs) i.e. CCLs with repetition rates from 10 GHz to 437 GHz and a total output power up to 50 mW per facet at room temperature [6, 9–14]. For these devices we have measured relative intensity noise (RIN) and phase noise of filtered

individual wavelength channels and the whole CCL spectrum, and their RF beating signal of the whole coherent comb [6, 15]. Unfortunately, the filtered individual wavelength channels of these lasers generally exhibit optical linewidths of the order of MHz [6, 15, 16]. As a consequence, those CCLs are not satisfactory for Tb/s (and higher) coherent networking systems [17].

Fig. 1. Spectrum of a C-band 25-GHz InAs/InP QD CCL at 380 mA and 20 °C

In order to improve net data transmission rates and spectral efficiency in optical coherent communication systems, researchers have put significant efforts to simultaneously reduce optical linewidth of each individual wavelength channel of CCLs [18,19]. A feed-forward heterodyne detection scheme [18] has been used to simultaneously reduce the optical linewidth of many comb lines from a mode-locked laser, but this method has difficulty producing a large set of comb lines (more than 20) simultaneously narrowed to a high degree (below a few hundred kHz). To circumvent the issues associated with the large linewidth a self-homodyne system has been tried where unmodulated carriers are transmitted as local oscillators along with the modulated signal [19]. Neither of these approaches actually address the linewidth of the laser source itself. On the other hand injection locking has been used to successfully reduce the linewidth of single wavelength sources such as semiconductor distributed feedback (DFB) lasers. This can be achieved by using a narrow wavelength master laser fed into the slave laser [20] or using the output of the DFB itself fed back to lock the laser through the use of an external cavity [21]. These approaches can allow a significant reduction in linewidth. Injection locking of a multiwavelength QD comb laser has been demonstrated using a single wavelength master laser, with some success, but resulted in a significant narrowing of the overall comb spectrum [22]. Simultaneous linewidth narrowing of each individual channel in a QD MLL [23, 24] has been demonstrated using resonant self-feedback from a free-space secondary cavity, but limited details are provided. Recently we have presented our preliminary experimental results in OFC 2018 [25]. In this paper, we have used an external cavity self-injection feedback locking (SIFL) system to simultaneously reduce the optical linewidth of each individual wavelength channel of an InAs/InP QD 25-GHz C-band CCL with the detailed explanation of how SIFL works with optical comb laser source. Linewidths are reduced from a few MHz down to less than 200 kHz over 39 channels without any significant change in the overall laser spectrum.

2. InAs/InP QD CCL, experimental set-up, results and discussions

The InAs/InP QD laser samples were grown by chemical beam epitaxy (CBE) on exactly (100) oriented n-type InP substrates. A 350 nm thick InGaAsP waveguide core contained 5 layers of QDs as the gain medium, surrounded by n- and p- type InP cladding layers. More detailed QD growth information is contained in reference [26]. This sample was fabricated into single lateral mode ridge waveguide lasers with a ridge width of 1.8 μm, and then cleaved to form a Fabry-Perot (F-P) laser cavity of length of 1693 μm. No facet coatings were used on those devices. The laser was driven with an ultra-low-noise battery powered laser diode driver, and tested on a temperature controlled heat sink. All measurements were

performed at a drive current of 380 mA and temperature of 20 °C. The performance of the QD CCL was characterized using an optical spectrum analyzer (Anritsu MS9740A), a 50 GHz PXA signal analyzer (Keysight Technologies Model N9030A), a tunable filter (Santec OTF-970), a 45 GHz IR photodetector (New Focus Model-1014), an optical autocorrelator (Femtochrome Research Inc FR-103HS), an Agilent N4371A relative intensity noise (RIN) measurement system, an OE4000 automated laser linewidth / phase noise measurement system (OEWaves Inc.) and power meter (ILX Lightwave FPM-8210H).

Fig. 2. (a) Optical linewidth of a selection of individual filtered channels from a QD CCL versus channel wavelength in the same operation condition without (black curve) and with (red curve) self-injection feedback locking system. (b) Experimental setup without feedback.

Fig. 3. A schematic of the ultra-narrow linewidth QD CCL using an external cavity self-injection feedback locking (SIFL) system. All components are polarization-maintaining.

Figure 1 shows an optical spectrum of the laser, the active length of 1693 μm giving a mode spacing of 25 GHz. The center wavelength is 1539.21 nm and the 3-dB bandwidth of the entire lasing spectrum is 11.85 nm, providing over 60 channels with an optical signal-to-noise ratio (OSNR) of more than 35 dB. From L-I-V curves a lasing threshold current of 48 mA, single facet slope efficiency of 0.13 mW/mA, and a series resistance of 1.46 Ohm were determined. The average output power was 42 mW per facet. Figure 2(a) shows the measured optical linewidth calculated from the frequency noise spectra (black square points) for a representative grouping of filtered wavelength channels from the laser versus channel wavelength over 39 channels. Figure 2(b) is an experimental setup without feedback. The laser output from the front facet of the QD CCL is coupled to an anti-reflection (AR) coated lensed polarization-maintaining (PM) single-mode (SM) fiber. A two-stage PM optical isolator is used to prevent any reflection back to the laser cavity from the measurement system. All measurements of the laser are characterized using this fiber output from the front facet. The optical linewidth of each individual channel is between 920 kHz and 4.51 MHz, which is not good enough for use as a laser source for Tb/s and beyond coherent optical

networking systems. The decrease in linewidth when going to longer wavelength is typical of the behaviour observed for all of our QD F-P lasers [6,15,16]. The possible reasons have been explained in reference paper [6].

In order to narrow the optical linewidth of all individual wavelength channels of the laser simultaneously, we have used an external cavity self-injection feedback locking (SIFL) system, shown schematically in Fig. 3. The external SIFL system is coupled to the back-facet of the laser using an AR-coated lensed PM SM fiber which is connected to port 1 of a PM optical circulator (OC). The output from port 2 of the PM OC passes through a PM variable optical attenuator (VOA), and back into port 3 of the PM OC. The circulating light is then reinserted at the back-facet of the laser cavity through port 1 of the PM OC and the AR coated lensed PM SM fiber. The physical length of this external SIFL cavity is approximately 8 meter. The above design provides an external cavity that is weakly coupled to the QD laser, where the degree of feedback is controlled using the VOA. The stable locking regimes of the feedback laser power, which is coupled back from the SIFL system, are from 5x10−4 to 5x10−5 of the laser output power from the rear facet in Fig. 3. We have increased the external cavity length by many meters and can obtain equivalent linewidth narrowing by adjusting both the VOA and laser drive current. Thus, while the external cavity length is an important parameter, the linewidth narrowing is not critically sensitive to the cavity length.

Fig. 4. A graphical comparison of the frequency noise spectra from two filtered wavelength channels of 1545.14 nm and 1542.34 nm with and without the external cavity SIFL system.

Fig. 5. Normalized RF beating signals of a 25 GHz QD CCL in the same operation conditions without (black curve) and with (red curve) the self-injection feedback locking (SIFL) system. Here the RBW and VBW of PXA Signal Analyzer N9030A are 30 Hz and 10 Hz, respectively.

Using this SIFL system at the rear-facet of the laser we observed a reduction of the optical linewidth of all laser lines by more than an order of magnitude, as shown in Fig. 2 (red star points). Optimisation of the feedback strength by tuning the VOA resulted in linewidth values of each individual channel being simultaneously reduced to between 12 kHz and 198 kHz over the 39 channels. All channels from 1537.55 nm to 1545.14 nm originally had optical linewidths above 920 kHz and are now well less than 200 kHz, varying from 1.3% to 4.4% of

the original linewidth. Even though the linewidths are dramatically reduced we observe no changes in the RIN values of each individual channel with and without the external SIFL system. Typical average-integrated RIN values of −130 dB/Hz are measured over frequency range from 10 MHz to 20 GHz. Unlike feedback schemes where just one wavelength is injected into the laser [22], we observed no changes in the shape of overall lasing spectrum.

Figure 4 give a graphical comparison of the frequency noise spectra from two filtered single channels with and without the external cavity SIFL system. The phase noise spectra clearly indicate that by using this SIFL system we can significantly reduce the phase noise in the high frequency range above 2 kHz, while the low frequency (< 2kHz) phase noise does not improve. Figure 5 shows the normalized RF beating signal spectra with and without the external cavity SIFL system when we have focused all comb lines onto a fast photodetector and have fed the electrical signals to an RF signal analyzer. A substantial reduction in 3-dB linewidth from 10 kHz to 300 Hz with feedback, and a corresponding drop in baseline level (−27 dB to −50 dB) is observed. The timing jitter of the QD CCL with the SIFL system has significantly been decreased because their timing jitter is directly related with their RF beating frequency’s linewidth according to the reference [27].

The QD laser is self-mode-locked, producing pulses at a repetition rate corresponding to the F-P mode spacing [9, 12, 13], resulting in a well-defined and strong phase relationship between the different lasing modes which is a necessary condition for narrowing all laser modes simultaneously. The QD laser RF beating signal with a linewidth of 10 kHz, Fig. 5, demonstrates this fixed mode spacing across the whole comb. For the external feedback system to work the mode spacing for the QD laser cavity must be equal to integer multiples of the external cavity mode spacing. This means that the total refractive index dispersion for the external cavity is required to be very small, the components used here resulting in a dispersion of less than 10−5 nm−1. The QD laser is also polarisation sensitive, so any feedback must not only have a fixed intensity and phase, but also a fixed polarisation. A variation in polarisation would result in an effective variation in the optical strength of the feedback. It is thus important to make sure that high quality polarisation preserving components are used in the external cavity. Under the above operation conditions each laser line feeds back with correct phase and amplitude to result in a narrowing of the original laser line in much the same way as for the single line DFB laser [21].

3. Conclusion

In conclusion, we have used an external cavity self-injection feedback locking (SFIL) system to simultaneously reduce the linewidth (phase noise) of each wavelength channels of an InAs/InP QD 25-GHz C-band coherent comb laser. The experimental results clearly show a remarkable reduction with all of the channels from 1537.55 nm to 1545.14 nm dropping from above 920 kHz to less than 200 kHz with feedback. While relative intensity noise (RIN) values of each individual wavelength channel did not change, a significant reduction in mode beating linewidth was observed with feedback. The resulting ultra-narrow linewidth QD CCL is suitable for Tb/s and beyond coherent communication systems.