Coupled-ring reflector in an external-cavity tunable laser

MIN REN, HONG CAI, LIP KET CHIN, K. RADHAKRISHNAN, YUANDONG GU, GUO-QIANG LO, DIM LEE KWONG, AND AI QUN LIU* School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore Institute of Microelectronics, A*STAR, 11 Science Park Road, Singapore 117685, Singapore e-mail: caih@ime.a‐star.edu.sg e-mail: guyd@ime.a‐star.edu.sg *Corresponding author: eaqliu@ntu.edu.sg

Miniaturized wavelength tunable lasers have attracted great interest, functioning as light sources in optical networks and sensing systems.To achieve a large tuning range, external cavity tunable lasers are used, which basically integrate a gain medium with one or more tunable "mirrors" in the external cavity [1].The tunable mirrors could be realized on multiple platforms, i.e., III-V, polymer, silica, and silicon [1][2][3][4][5].Among all these possibilities, silicon is particularly promising for tunable lasers, benefiting from the high refractive index, cost efficiency, and compatibility for mass production.Based on the silicon platform, microelectromechanical systems (MEMS) technology has been used for external-cavity tunable diode lasers to minimize laser size and improve stability [6][7][8].However, the employment of a free-space external cavity leads to large optical loss due to beam divergence, deteriorating cavity efficiency to below 10%, and degrading the output power and the side-mode-suppression ratio (SMSR).
Nano-silicon-photonic circuits have been utilized for guiding light in external-cavity tunable lasers [9][10][11].Wavelength selective silicon-photonic elements, such as ring resonators, gratings, and Mach-Zehnder interferometers, have been continuously developed [12,13].Among these elements, ring resonators are superior for a tunable laser due to their narrow resonance bandwidth and high quality factor [14,15].Two InP-based ring resonators are separated at different sides of the gain section in a tunable laser to serve as wavelength filters [16].To avoid gain absorption at the filtering section, the ring resonators have been positioned at the same side of the gain section [17].However, such wavelength filters do not provide a reflective function, thus creating a need for additional mirror/reflective facets.A pair of mutually coupled-ring resonators has been proposed as a wavelength selective reflector [18].However, such a reflector lacks tunability and suffers from high optical loss.The relatively low transmission loss (0.5-1.5 dB/cm) of a nano-silicon waveguide is perfect for high external-cavity efficiency.The thermo-optic effect was adopted for efficient refractive index tuning in silicon ring resonators [19].However, this effect is easily affected by environmental temperature fluctuation with limited tuning accuracy and speed (submicrosecond).In complement, the free-carrier dispersion effect, which enables independence from temperature fluctuations with improved tuning accuracy and high tuning speed, can be applied.
In this Letter, a pair of coupled-ring resonators acting as a reflector to form the external cavity of an integrated siliconphotonic tunable laser is demonstrated.This is, to our best knowledge, the first demonstration of a silicon coupled-ring reflector controlled by complementary wavelength tuning schemes, providing high external-cavity efficiency (22%) and flexible wavelength tuning of the laser.The thermo-optic effect enables a wide 40 nm wavelength tuning range, and the free-carrier dispersion effect ensures fine-tuning with 2 pm accuracy.This high tuning accuracy is critical for enhancing tunable laser performance, such as output power and the SMSR.
Letter Figure 1(a) shows the schematic of the monolithic externalcavity tunable laser.It consists of a gain chip as the internal cavity and a pair of coupled-ring resonators connected with the gain chip through the nano-silicon bus waveguide in the external cavity.The coupled-ring tunable resonators serve as a reflector, replacing one cleaved facet in the conventional Fabry-Perot laser.The reflectivity is controllable by adjusting properties such as the radii, the effective refractive index, and the coupling coefficients ringring and bus-ring.Ring resonator 1 (Ring 1), with a refractive index that is thermally adjustable for potential continuous wavelength tuning, serves as the reference ring.Ring resonator 2 (Ring 2) is used to control the lasing wavelength.A p-i-n junction is formed across Ring 2, and a microheater is patterned on top of the ring waveguide and isolated by a SiO 2 layer.The specific wavelength that meets the resonance of both ring resonators, which is also the lasing wavelength, has the highest reflectivity.The wavelength is tuned by controlling the effective refractive index of the ring waveguide.The gain chip and the nano-silicon photonic circuits are optically connected by a nano-silicon inversed taper waveguide to obtain mode-size matching.The equivalent cavity model is illustrated in Fig. 1(b).The gain chip forms the internal cavity with a cavity length of n int L int .The left facet has a reflectivity of 0.1, while the right facet is antireflective coated, leading to approximately zero reflectivity.With the bus waveguide and the pair of coupled-ring resonators, an external cavity with a cavity length of n out L out n bus L bus n ring L ring is formed, leading to an effective reflectance r eff n ring ; λ.The longitudinal modes of the equivalent cavity satisfies the equation 2n int L int n out L out mλ.At the initial condition whereby the effective refractive index in the nano-silicon ring waveguide is n ring , the effective reflectivity of the external cavity is expressed as where t is the coupling coefficient between the gain chip and the nano-silicon waveguide, λ is the optical wavelength, and r c n ring ; λ is the reflectivity of the coupled-ring reflector, which is calculated via the transfer matrix method [18].r c n ring ; λ indicates the central resonance λ 0 , as well as the loss margin between λ 0 and its adjacent reflection maxima, whereby the tuning of the refractive index of the ring waveguide n ring results in the tuning of r c .L bus affects the optical path length of the laser cavity and thus the mode spacing.The total optical path length of the laser cavity is ∼13.3 mm, and the longitudinal mode spacing is ∼90 pm.The bandwidth at central resonance is 100-200 pm, which is compatible with the longitudinal mode spacing.The maximum externalcavity efficiency is obtained only when the mth longitudinal mode matches the central resonance λ 0 .Under this situation, the maximum SMSR and output power are achieved for single-mode lasing.To meet this condition, a 10 pm tuning accuracy (1 order smaller than the resonance bandwidth) is required to refine the position of λ 0 .To achieve continuous tuning, the free spectral range of the entire cavity has to be adjustable for phase matching, and the central reflective wavelength has to be tuned simultaneously.Phase matching is obtained by changing the effective refractive index of both ring resonators because the effective optical length of the ring resonators is part of the cavity length.Continuous tuning is achieved by simultaneously heating the two ring resonators.In addition, an accurate match between the central reflective wavelength and the longitudinal mode is obtained by the complementary thermal and free-carrier tuning of Ring 2, thus guaranteeing high SMSR.
The tuning of n ring results in the tuning of λ 0 , and, thus, the lasing wavelength.Based on the Vernier effect, the wavelength tuning amount Δλ tune is expressed as where R is the radius of Ring 2, ΔR 2.4 μm is the radius difference, p ∼ 0.9 is the effective tuning ratio, and Δn si is the change of the refractive index, which is expressed as Here, V T and V PIN are the thermal voltage and the p-i-n junction voltage, respectively, V 0 ≈ 0.7 V is the switch on voltage of the silicon based p-i-n junction, and A and B are constants.Thus, the effective index tuning coefficient of the thermo-optic is dn si dV T 2AV T , which increases with thermal voltage V T , while the tuning coefficient of the free-carrier injection and B is 2-3 × 10 −4 ∕V, based on the experiments.The effective index tuning coefficient is j dn si dV T j > 20j dn si dV PIN j, when V T > 1 V.A 40 nm wavelength tuning range requires the amount of Δn si max > 4 × 10 −3 , obtainable by thermo-optic tuning; a 10 pm wavelength tuning accuracy requires the step of Δn si min < 3 × 10 −6 , obtainable by free-carrier injection.
The silicon-photonic tunable laser is fabricated on a siliconon-insulator (SOI) wafer with a 220 nm thick structure layer.The gain chip, with dimensions of 250 μm × 250 μm × 100 μm, is flip-chip bonded onto the fabricated SOI chip.The radii of Ring 1 and Ring 2 are 62 and 59.6 μm, respectively.For high reflectivity and wavelength selectivity, the coupling coefficients for the ring-ring and bus-ring coupling are chosen as 0.009 Fig. 1.Schematic illustration of (a) the integrated silicon-photonic tunable laser using a pair of coupled-ring resonators, with its lasing wavelength electrically tuned by the complementary free-carrier dispersion and thermo-optic effects and (b) the equivalent model of the externalcavity tunable laser with a coupled-ring reflector.and 0.16, respectively.The width of the taper waveguide is gradually decreased from 3 μm to 450 nm, and the length of the taper waveguide is 200 μm.The nano-silicon waveguides, for both the bus and the ring resonators, have dimensions of 450 nm in width and 220 nm in height.The bus waveguide length L bus is 550 μm.The SEM image of the hybrid integrated tunable laser is shown in Fig. 2. The misalignment between the gain chip and the silicon waveguide is controlled within 300 nm in the lateral direction, and 100 nm in the vertical direction.The coupling gap is 200 nm between the ring and the bus waveguide and 320 nm between the two ring resonators.In the experiments, the packaged tunable laser is placed on a submount, connecting with a thermoelectric cooler.Its working temperature is kept at 25°C.A broadband light source with a power density of −16 dBm per 0.1 nm is coupled into the coupled-ring resonators through an optical circulator and two lens fibers.The transmission and reflection spectra are measured by an optical spectrum analyzer (OSA, Advantest Q8384).The threshold of the tunable laser is ∼150 mA and the gain diode is electrically pumped at 185 mA.For wavelength tuning, the thermal voltage applied to the microheater V T and the junction voltage V PIN are tuned within 0 to 1.45 V and 0.7 to 1.2 V, respectively, with minimal voltage step of 0.01 V.
Figure 3 shows the transmission and reflection spectra of the coupled-ring reflector.The central wavelength is λ 0 1564.18nm with a bandwidth of 160 pm.Based on the optical intensity level, the reflection peak is lowered by 2.9 dB.Since the optical circulator has a 1.2 dB insertion loss, the reflectivity of the coupled-ring reflector is estimated to be 70%.The optical loss of the nano-silicon waveguide is 1.5 dB/cm, and the coupling loss between the active and the passive waveguides is approximately 2.5 dB; thus, the external cavity efficiency of the tunable laser is estimated to be 22%, which is threefold higher compared to the free-space MEMS tunable lasers [8].
Figure 4(a) shows the accurate tuning of the lasing wavelength based on free-carrier injection.The thermo-optic tuning accuracy is degraded, accompanying the increase of tuning efficiency at high V T .For example, when Ring 2 is operated at V T 1.36V and V PIN 0, the lasing wavelength is at 1529.80 nm.Based on the minimal voltage step of 0.01 V, the thermo-optic tuning accuracy is 240 pm.This accuracy is unable to refine the position of the longitudinal mode within the ∼160 pm resonance band.The position of the center of the resonance band λ 0 is biased from the targeted longitudinal mode.Consequently, the SMSR and the output power are limited at 26.5 dB and −2.5 dBm, respectively.Although the thermal tuning accuracy can be improved by using a driving circuit with smaller voltage steps, the accuracy and stability are easily affected by the tiny fluctuation of environmental temperature.Reliable accurate tuning is demonstrated using free-carrier injection, which refines the position of λ 0 .When V T is kept as a constant at 1.36 V and V PIN is tuned from 0.7 to 0.85 V, the lasing wavelength has a small redshift of 30 pm, and the tuning accuracy is 2 pm per 0.01 V.As a result, the SMSR and the output power are refined to 40.5 dB and 3 dBm, respectively.In addition to the advantage of high tuning accuracy, the free-carrier dispersion effect also has a fast switching speed, as shown in Fig. 4(b).To test the switching speed, the p-i-n junction works at a bias voltage of 1.0 V and a modulation voltage of 1.6 V.The modulation frequency is set to 0.5 GHz.The rise   and fall times are 0.78 ns and 0.72 ns, respectively, which are 10 3 -fold faster as compared to the thermo-optic response.
The free-carrier injection is necessary for accurate phase matching when V T > 1.20 V.The mode map of the SMSR, as a function of both V T and V PIN of Ring 2, is shown in Fig. 5(a).The SMSR is optimized to above 40 dB when V PIN is tuned from 0.7 to 1.0 V.The wavelength tuning is discrete with a 1.62 nm wavelength spacing when the effective refractive index of Ring 1 is fixed and Ring 2 is tuned.Continuous wavelength tuning is obtained when both ring resonators are made tunable.A mode map of the lasing wavelength, as a function of the thermal voltages applied on both ring resonators, is shown in Fig. 5(b).A series of single-mode lasing spectra is combined and shown in Fig. 6.Benefiting from the fine tuning accuracy by free-carrier injection, the SMSR is kept above 40 dB within the wavelength range from 1522.54 to 1552.90 nm, which covers 75% of the accessible spectrum, and the maximal output power is 3 dBm.The SMSR is reduced (minimum 37.7 dB) due to the degraded gain level at the edge of the accessible spectrum, and the power is reduced to −1.5 dBm.In addition, due to the accurate and fast tuning properties, the free-carrier effect makes quick compensation possible to a lasing wavelength shift caused by temperature change or gain current fluctuation.
In conclusion, a silicon-photonic tunable laser using a pair of coupled-ring resonators as a reflector is demonstrated.With the central reflective wavelength and the phase matching being simultaneously controlled by complementary free-carrier dispersion and thermo-optic effects, the low-loss coupled-ring reflector guarantees a 22% external-cavity efficiency, 41.6 nm tuning range, 2 pm tuning accuracy, and less than 1 ns response speed.The wavelength tuning through free-carrier injection ensures 40 dB SMSR and 2 mW output power, as well as quick wavelength compensation capability.The tunable laser provides potential applications in high-speed optical networks, optical sensors, and on-chip optoelectronic systems.

Fig. 2 .
Fig.2.SEM images of the integrated silicon-photonic tunable laser using a pair of coupled-ring resonators.The gain chip is bonded by flip-chip packaging processes.

Fig. 3 .
Fig. 3. Transmission and reflection spectra of the coupled-ring reflector.The reflection spectrum is measured using a fiber optical circulator with a 1.2 dB insertion loss.

Fig. 4 .
Fig. 4. Experimental results of (a) the demonstration of accurate wavelength tuning due to free-carrier injection, denoted by the improvement of the SMSR and the output power and (b) the response speed of the free-carrier dispersion effect; the rise and fall times are 0.78 and 0.72 ns, respectively.

Fig. 5 .
Fig. 5. Mode maps of (a) the SMSR as a function of V T and V PIN applied on Ring 2 and (b) the lasing wavelength as a function of the thermal voltages applied on the two ring resonators.

Fig. 6 .
Fig.6.Optical spectra of a series of single-frequency lasing ranging from 1564.18 to 1522.54 nm; it obtains over 40 dB SMSR and maximal 2 mW output power due to the fine tuning of the free-carrier injection.