We present a monolithic integrated Raman silicon laser based on silicon-on-insulator (SOI) rib waveguide race-track ring resonator with an integrated p-i-n diode structure. Under reverse biasing, we achieved stable, single mode, continuous-wave (CW) lasing with output power exceeding 30mW and 10% slope efficiency. The laser emission has high spectral purity with a measured side mode suppression exceeding 70dB and laser linewidth of <100 kHz. This laser architecture allows for on-chip integration with other silicon photonics components to provide a highly integrated and scaleable monolithic device.
©2006 Optical Society of America
Silicon photonics technology has made rapid progress and achieved several key milestones in recent years; it offers low-cost optoelectronic solutions for applications ranging from telecommunications to chip-to-chip interconnects and to emerging areas such as optical sensing and bio-medical applications [1–6]. In particular, stimulated Raman scattering has been successfully used to demonstrate amplifiers [7–13] and lasers [14–18] in silicon. In these proof-of-principle demonstrations, the laser cavities have been either external  or formed by applying multi-layer optical coatings onto the chip facets [15, 16], which do not allow monolithic integration and scalability. Here we report a fully monolithic integrated ring cavity Raman silicon laser. The laser cavity is based on a race-track ring resonator with an integrated p-i-n structure [17, 18]. By revere biasing the p-i-n diode to reduce the nonlinear losses due to two-photon absorption (TPA) induced free carrier absorption (FCA), we have achieved stable, single mode, CW lasing, with output power exceeding 30mW and a 10% slope efficiency. The measured side mode suppression ratio (SMSR) exceeds 70dB and the laser linewidth is <100 kHz. This superior spectral purity can provide unique advantages for applications in coherent optical communications, spectroscopy, metrology, and optical sensing, and this laser design allows for on-chip integration with other silicon photonics components to provide a highly integrated and scaleable monolithic device.
2. Device design and fabrication.
The monolithic integrated cavity is constructed from a low-loss silicon-on-insulator (SOI) rib waveguide forming a race-track shaped ring resonator on a single chip (Fig. 1). A bus waveguide is connected with the ring cavity via a directional coupler which couples both pump and signal laser light into and out of the cavity. The coupling ratio depends on input wavelength and polarization and can be varied by changing the gap and/or length of the coupler . In our experiment, we varied the coupler lengths to obtain different coupling ratios for pump and lasing wavelengths. The design is based on the dispersion and polarization dependence of the coupling coefficient. Beamprop  was used to simulate the propagation of coupled waveguides to obtain desired coupling ratio. To make the most efficient use of the pump and achieve low lasing threshold, we aimed at critical coupling for pump light at 1550 nm and under coupling for the Stokes wavelength at 1686 nm. The coupling ratio and optical propagation loss of the waveguide are determined experimentally by analyzing the transmission spectrum of the ring resonators .
The silicon rib waveguide is fabricated on the (100) surface of an undoped SOI substrate using standard photolithographic patterning and reactive ion etching techniques. A cross-section scanning electron microscope image and a schematic for the directional coupler region of a typical p-i-n laser cavity are shown in Fig. 2. The rib waveguide dimensions are: rib width (W) ~ 1.5μm, height (H) ~1.55μm and etch depth (h) ~0.7μm. The gap (d) between the two waveguides in the coupler is ~0.7 μm and the coupling length varies between 700 and 1100 um to obtain different coupling ratios ranging 0 - 100% for both pump and signal wavelengths. The effective core area  of the waveguide is calculated to be ~1.6 μm2. The total length of the ring cavity is 3 cm and the bend radius is 400 μm (Fig. 1). The straight sections of the waveguide are oriented along the  crystallographic direction.
A p-i-n diode structure was designed to reduce the nonlinear optical loss due to TPA induced FCA [23–25]. The p-i-n structure was formed by implanting boron and phosphorus in the slab on either side of the rib waveguide with a doping concentration of ~1×1020 cm-3 to form p- and n-regions as highlighted in Fig. 1. Careful processing was done to optimize the coupler including filling the gap region with boron phosphorous silicon glass (BPSG) film thermally flowed to insure no voids were present in the coupler region. The separation between the p- and n-doped regions was designed to be ~6 μm except in the coupler region which has p- and n- separation of ~8 μm to accommodate the two waveguides. Ohmic contacts were formed by depositing aluminum films on the surface of the p- and n-doped regions. The separate sections of p- and n-doped regions to accommodate the coupler structure are interconnected via metal traces (not shown in Fig. 1). The doped regions and the metal contacts at the designed separation had negligible effect on the propagation loss of the waveguide because the optical mode is tightly confined in the waveguide. The average linear optical transmission loss of the race-track ring resonator was determined to be 0.6 ± 0.1 dB/cm by measuring its transmission spectrum using a tunable laser and the Q of the cavity was found to be ~6×105.
3. Experimental results and discussions
At higher input powers, the effect of TPA and FCA become significant. The pump power along the ring cavity in the propagation direction z can be described by
where αp is the linear loss coefficient for the pump, β = 0.5 cm/GW  is the TPA coefficient, σp = 1.45×10-17 cm2 is the FCA cross section  at the pump wavelength of 1550 nm, Ep is the photon energy, and τeff is the effective carrier lifetime which was measured to be ~1.0 ns at a reverse bias of >10V using the method described in ref. .
Neglecting pump depletion at pump powers below lasing threshold, the round trip gain at the Stokes wavelength in the ring cavity can be calculated by integrating the net gain (Raman gain minus loss) along the propagation path using
where gr = 9.5 cm/GW is the Raman gain coefficient , σs = 1.71×10-17 cm2 is the FCA cross section  at the Stokes wavelength of 1686 nm. Using finite element method, we can numerically derive the evolution of the pump field inside the cavity and obtain the net round trip gain for our laser cavity to determine the threshold. On resonance, the relation between the incident power and the power inside the cavity is described by
where Kp is power coupling ratio, A is the total round trip loss and L is the cavity length. For example, at coupling ratios of 5% and 10% for the lasing wavelength, the laser threshold is expected to be 150 mW and 180 mW respectively with a fixed 40% coupling for the pump wavelength.
Figure 3 is a schematic of the ring laser experimental setup. A CW external cavity diode laser at 1550 nm amplified by an erbium-doped fiber amplifier was used as the pump. The pump beam passes through a polarization controller followed by a thin-film-based WDM filter and is coupled into the waveguide cavity by a lensed fiber. The Raman laser output at 1686 nm is coupled back into the same lensed fiber, and separated from the pump by the WDM filter followed by an additional long wavelength pass filter before being detected by a power meter or optical spectrum analyzer. Note that the laser beam exits from both sides of the cavity. The coupling loss between the lensed fiber and the waveguide was measured to be ~ 4 dB and the total insertion loss of the WDM filter and the long-pass filter is ~1.0 dB.
Figure 4 plots the silicon ring laser output power versus the input pump power (Iinc) coupled into the bus waveguide as depicted in Fig. 1 for 4 ring laser devices with varying coupling ratios for pump and laser wavelengths. In the experiment, the pump beam polarization is adjusted with a polarization controller and its wavelength is fine tuned to the cavity resonance to take advantage of the cavity enhancement  to maximize the laser output. The silicon ring laser frequency is 15.6 THz red-shifted from the pump laser wavelength. As can be seen from Figure 4, the lasing threshold and output power depend on the coupling ratio for the pump and signal wavelengths. For a given pump coupling, lower coupling for the lasing wavelength corresponds to a lower threshold, but lower output power. In contrast a higher coupling ratio corresponds to a higher threshold and higher output power. The highest output power measured for device D is above 30mW with a slope efficiency of ~10%. The lowest silicon ring lasing threshold measured for device A is 144mW. The measured lasing thresholds agree reasonably well with that predicted by modeling (see inset in Fig. 4) within 10%. All these curves were taken at a reverse bias of 10V. Figure 5 shows the effect of reverse bias voltage on the lasing performance of device B at a pump power of 250mW. The laser output power increases with increasing reverse bias voltage. This is expected as the effective carrier lifetime becomes shorter, resulting in lower nonlinear loss and higher net gain . Lasing can be achieved with a reverse bias as low as ~2.3 V.
The spectrum of the laser output is measured with an optical spectrum analyzer (Ando AQ6317B). Figure 6 shows the silicon ring laser (device C) spectrum measured at a coupled pump power of ~300mW and a reverse bias of 10V. As shown, the laser has a single frequency output and the side mode suppression ratio is more than 70dB. Since the laser linewidth could not be resolved with the grating based optical spectrum analyzer, we measured the line width of the laser using a delayed self-heterodyne method . A fiber optic interferometer (Agilent 11980A) was used and the laser output was phase modulated at 500 MHz. Figure 7 shows the beat signal at the output of the interferometer measured with a fast photo-detector and an electrical spectrum analyzer. The 3 dB linewidth of the beat signal is 160 kHz which corresponds to a laser linewidth of 80 kHz assuming a Lorentzian line shape.
Further improvements of the silicon ring laser performance include reducing the waveguide loss and optimizing the coupler, cavity length, and p-i-n diode design. Based on our modeling, we expected the lasing threshold to be < 50mW with a waveguide loss of 0.3dB/cm which we were able to achieve previously [11, 15, 16]. Unlike laser cavity configurations reported earlier [14–16], this design allows dimension scalability and on-chip integration with other functional components. Reducing the waveguide cross sectional dimensions and/or by introducing impurities into the waveguides  in combination with the p-i-n diode structure could reduce the carrier lifetime to below 1ns, thus increasing the laser output power. In addition, other cavity configurations can be considered to minimize the device size and increase efficiency. A monolithic integrated cavity structure based on photonic band gap has been proposed where a sub mW lasing threshold is predicted .
The realization of monolithic integrated Raman silicon lasers represents another milestone towards producing practical and truly monolithic silicon based photonic devices. The demonstrated high SMSR and narrow linewidth of the Raman silicon laser could open entirely new potential applications for these type of lasers in areas such as coherent optical communications, spectroscopy, metrology, and optical sensing where high spectral purity is of particular value. Similar to fiber Raman ring or micro-cavity lasers [29, 30], cascaded lasing is also possible using these silicon ring cavities. This would allow extending of the lasing wavelength into the mid infrared region where a suitable room temperature semiconductor laser is in high demand.
The authors thank N. Izhaky, A. Alduino, D. Tran, K. Callegari, R. Gabay and A. Ugnitz, for assistance in device fabrication and sample preparation; G. T. Reed, and J. E. Bowers for helpful discussions.
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