Slab-coupled Optical Waveguide Amplifier Research Articles

High-Power (>300 mW) On-Chip Laser With Passively Aligned Silicon-Nitride Waveguide DBR Cavity

We demonstrate a high-power on-chip 1550-nm laser implemented by passive flip-chip integration of a curved-channel, double-pass InGaAsP/InP slab-coupled optical waveguide amplifier (SCOWA) onto a photonic integrated circuit (PIC) containing a silicon nitride (SiN) waveguide and distributed Bragg reflector (DBR) grating. The combined chip-scale SCOW external cavity laser (SCOWECL) has single mode emission with 312 mW of optical power at a drive current of 2.5 A, and exhibits a side mode suppression ratio (SMSR) of 55 dB, peak photon conversion efficiency (PCE) of 10%, low relative-intensity noise (RIN) of ~160 dB/Hz, and an integrated linewidth of 192 kHz. Additionally, we demonstrate a multi-wavelength SCOWECL array comprised of four SCOWAs coupled to SiN-waveguide DBR gratings. The four-element array generates 80 mW per channel when the SCOWAs are driven in parallel (1.25 A/channel) with ~1-nm wavelength spacing centered at 1533 nm. We describe the fabrication and hybrid integration processes. The measured SCOWA-to-waveguide coupling loss is estimated to be 1.6 +/-1.0 dB which agrees well with the simulation.

Demonstration of a highly stable 10 GHz optical frequency comb with low timing jitter from a SCOWA-based harmonically mode-locked nested cavity laser.

An optical frequency comb with mode spacing of 10GHz operating in the c-band is produced from a harmonically mode-locked laser using a slab-coupled optical waveguide amplifier device with a fiber-coupled external cavity. An intracavity Fabry-Perot etalon serves as a high finesse optical filter for supermode suppression and as the reference for cavity length stabilization using a multi-combline Pound-Drever-Hall setup. The Allan deviation of a single optical combline near 193.4THz is measured via a heterodyne beat with a cavity stabilized cw laser and reaches a minimum fractional frequency deviation of 3×10-13 at τ=30 ms. In addition, the phase noise of the photodetected pulsed output of the laser shows a timing jitter of <23 fs integrated from 1Hz to the Nyquist frequency.

Inductively coupled plasma reactive ion etching of GaAs wafer pieces with enhanced device yield

Inductively coupled plasma reactive ion etching (ICP-RIE) is used in the fabrication of GaAs slab-coupled optical waveguide (SCOW) laser and amplifier devices in order to prepare etched-ridge-waveguide surface features. The processing of GaAs wafer pieces (less than full wafers) requires mounting these samples on a ceramic or silicon carrier wafer by means of a thermally conductive mounting paste to improve thermal contact between the GaAs and carrier wafer. However, use of a mounting paste requires additional postetch handling of samples, including mechanical clean-up and multiple solvent cleaning steps. Insufficient paste removal can lead to unwanted surface contamination and film adhesion issues during subsequent sample processing. Massachusetts Institute of Technology Lincoln Laboratory has developed an ICP-RIE process for GaAs wafer pieces that eliminates the use of mounting paste. This process features time-limited thermal management during etching, which is essential to maintain predictable etch rates along with suitable etched surfaces and satisfactory sidewall quality. Utilizing this simplified etch process for SCOW fabrication has resulted in greatly reduced film adhesion failures and a dramatic improvement in device yield.

Chamber conditioning process development for improved inductively coupled plasma reactive ion etching of GaAs/AlGaAs materials

Dry-etch tool preparation, which is critical to ensuring reproducible etch conditions, is particularly important in multiple-user, multiple-process-tool settings. A reproducible dry-etch process has been developed, utilizing inductively coupled plasma reactive ion etching (ICP-RIE) of GaAs and AlGaAs materials, for the fabrication of ridge structures in slab-coupled optical waveguide semiconductor diode lasers and amplifiers. A commercial ICP-RIE system was used, configured with aluminum-oxide-coated chamber components and a SiCl4/Cl/Ar etch gas mixture. Passivation of etch chamber component surfaces by preconditioning or “seasoning” contributed to a chemically stable etch environment as monitored by tracking the GaAs etch rate. The etched areas and sidewall profiles obtained using this process were smooth, and run-to-run etch depth control was ±2% of the desired target depth of ∼1.25 μm. Energy dispersive analysis x-ray of the etch chamber surfaces before and after chamber conditioning is reported.

Ultralow Noise, Etalon Stabilized, 10-GHz Optical Frequency Comb Based on an SCOW Amplifier

A slab-coupled optical waveguide amplifier (SCOWA) is used to construct a 10-GHz harmonically modelocked laser with an intra-cavity etalon that serves as a high-finesse spectral filter. The laser output is a frequency comb that is actively stabilized to the transmission peaks of the etalon through a Pound-Drever-Hall scheme. The linewidth of the individual comb-lines is ~1 kHz and the frequency instability <;300 kHz over 60 s. The output power exceeds 20 mW due to the high saturation power of the SCOWA. The residual timing jitter of the pulse-train is ~1.9 fs (1 Hz to 100 MHz). A much broader comb (~10 nm) is generated after dispersion compensation of the cavity and the output pulses are compressible to <;1 ps. A comparison of the timing jitter of the all-anomalous dispersion and dispersion compensated cavities is presented along with a previous result with a similar architecture.

High efficiency coherent beam combining of semiconductor optical amplifiers

We demonstrate 40 W coherently combined output power in a single diffraction-limited beam from a one-dimensional 47-element array of angled-facet slab-coupled optical waveguide amplifiers at 1064 nm. The output from each emitter was collimated and overlapped onto a diffractive optical element combiner using a common transform lens. Phase locking was achieved via active feedback on each amplifier's drive current to maximize the power in the combined beam. The combining efficiency at all current levels was nearly constant at 87%.

Packaged, High-Power, Narrow-Linewidth Slab-Coupled Optical Waveguide External Cavity Laser (SCOWECL)

We report the demonstration of an InGaAlAs/InP quantum-well, high-power, low-noise packaged semiconductor external cavity laser (ECL) operating at 1550 nm. The laser comprises a double-pass, curved-channel slab-coupled optical waveguide amplifier (SCOWA) coupled to a narrow-bandwidth (2.5 GHz) fiber Bragg grating passive cavity using a lensed-fiber. At a bias current of 4 A, the ECL produces 370 mW of fiber-coupled output power with a Voigt lineshape having Gaussian and Lorentzian linewidths of 35 and 1 kHz, respectively, and relative intensity noise <; -160 dB/Hz from 200 kHz to 10 GHz.

Noise Figure of Watt-Class Ultralow-Confinement Semiconductor Optical Amplifiers

We investigate the noise figure (NF) of high-power semiconductor InGaAsP optical amplifiers (SOAs) based on the slab-coupled optical waveguide (SCOW) concept having both ultralow optical confinement (Γ ~ 0.5%) and low optical loss (αi ~ 0.5 cm-1). At 1550 nm and 5-A current bias, the NF of SCOW amplifier (SCOWA) is 5.5 dB, and the small-signal gain and saturation output power are 13 dB and 0.8 W, respectively. A minimum NF of 4.5 dB is achieved at 2-A bias. These NF results represent the lowest reported for a packaged SOA. Using the measured NF, the population inversion factor (nsp) of the SCOWA was also estimated. The derived nsp values indicate that intervalence band absorption loss, carrier heating, and quasi-bound higher order modes may ultimately limit the noise performance of InGaAsP SOAs.

Packaged 1.5-$\mu$m Quantum-Well SOA With 0.8-W Output Power and 5.5-dB Noise Figure

We report the demonstration of a lensed-fiber-pigtailed InGaAsP-InP quantum-well semiconductor optical amplifier based on the slab-coupled optical waveguide (SCOW) concept. At a 5-A bias current and a wavelength of 1540 nm, the packaged SCOW amplifier (SCOWA) exhibits a record 0.8-W saturation output power, 13.8-dB small-signal gain, 5.5-dB noise figure, and a maximum electrical-to-optical conversion efficiency of 11%. The estimated coupling efficiency between the large (5.6 times 7.5 mum), fundamental SCOWA mode and the lensed fibers (6.5-mum spot size) is 90%.

Intracavity dispersion effect on timing jitter of ultralow noise mode-locked semiconductor based external-cavity laser

We report the generation of optical pulse trains with 380 as of residual timing jitter (1 Hz-1 MHz) from a mode-locked external-cavity semiconductor laser, through a combination of optimizing the intracavity dispersion and utilizing a high-power, low-noise InGaAsP quantum-well slab-coupled optical waveguide amplifier gain medium. This is, to our knowledge, the lowest residual timing jitter reported to date from an actively mode-locked laser.

Ultrafast nonlinearities and gain dynamics in high-power semiconductor amplifiers

The limits imposed by two-photon absorption and free-carrier absorption on the gain and output powers of an InGaAsP∕InP slab-coupled optical waveguide amplifier with a confinement factor of Γ=0.5% were studied. The two-photon absorption coefficient and the induced free-carrier absorption cross section were measured to be 65cm∕GW and 7×10−17cm2, respectively. The effects of two-photon absorption begin to limit the gain significantly for pulses shorter than 40ps. The carrier recovery times were observed to vary between 390 and 160ps for 1–4A bias currents, and the short-pulse saturation fluence of the gain was determined to be 1.4mJ∕cm2.

Continuous-wave two-photon absorption in a Watt-class semiconductor optical amplifier

We report the observation of photoluminescence produced by the recombination of free carriers generated via continuous-wave (CW) two-photon absorption (TPA) in a packaged, low-confinement (Gamma approximately 0.5%) InGaAsP/InP quantum-well slab-coupled optical waveguide amplifier (SCOWA) having a saturation output power of 0.8 W and 1/e-mode-field diameters of 5 x 7 microm. Photoluminescence power measured at the wavelength corresponding to the bandgap wavelength of the SCOWA's InGaAsP waveguide (lambda(G) approximately 1040 nm) exhibits a quadratic dependence on the amplifier's 1540-nm output power. Comparison between measured and simulated CW gain saturation data reveals that the combination of TPA and TPA-generated free-carrier absorption (FCA) limits the CW output intensity of high-power, low-confinement semiconductor optical amplifiers and semiconductor lasers.

Ultralow-noise mode-locked optical pulse trains from an external cavity laser based on a slab coupled optical waveguide amplifier (SCOWA)

We report the generation of optical pulse trains with 8.5 fs timing jitter (10 Hz to 10 MHz) from a mode-locked semiconductor laser, with a slab coupled optical waveguide amplifier used as the gain element. This is, to our knowledge, the lowest residual timing jitter reported to date from an actively mode-locked laser.