Slab-coupled Optical Waveguide 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.

Dielectric Coating Thermal Stabilization During GaAs-Based Laser Fabrication for Improved Device Yield

The quality and yield of GaAs-based ridge waveguide devices fabricated at MIT Lincoln Laboratory were negatively impacted by the random lot-to-lot appearance of blisters in the front-side contact metal. The blisters signaled compromised adhesion between the front-side contact metal, underlying SiO2 dielectric coating, and semiconductor surface. A thermal-anneal procedure developed for the fabrication of GaAs slab coupled optical waveguide (SCOW) ridge waveguide devices stabilizes the SiO2 dielectric coating by means of outgassing and stress reduction. This process eliminates a primary source of adhesion loss, as well as blister generation, and thereby significantly improves device yield. Stoney’s equation was used to analyze stress-induced bow in device wafers fabricated using this stabilization procedure. This analysis suggests that changes in wafer bow contribute to the incidence of metal blisters in SCOW devices.

Investigation of electrochemical etch differences in AlGaAs heterostructures using Cl2 ion beam assisted etching

A deeply etched, anisotropic 45° and 90° mirror technology is developed for AlxGa1−xAs heterostructures using a Cl2 ion beam assisted etching system. When etching vertically, using a conductive low-erosion Ni mask, electrochemical etch differences between layers with various Al mole fractions caused nonuniform sidewall profiles not seen in semi-insulating GaAs test samples. These variations, based on alloy composition, were found to be negligible when etching at a 45°. A Si3N4-Ni etch mask is designed in order to electrically isolate charge buildup caused by the incoming Ar+ ion beam to the Ni layer, preventing conduction to the underlying epitaxial layers. This modification produced smoothly etched facets, up to 8 μm in depth, enabling fabrication of substrate–surface-emitting slab-coupled optical waveguide lasers and other optoelectronic devices.

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.

Merged beam laser design for reduction of gain-saturation and two-photon absorption in high power single mode semiconductor lasers

In this paper we report a method to overcome the limitations of gain-saturation and two-photon absorption faced by developers of high power single mode InP-based lasers and semiconductor optical amplifiers (SOA) including those based on wide-waveguide or slab-coupled optical waveguide laser (SCOWL) technology. The method is based on Y-coupling design of the laser cavity. The reduction in gain-saturation and two-photon absorption in the merged beam laser structures (MBL) are obtained by reducing the intensity of electromagnetic field in the laser cavity. Standard ridge-waveguide lasers and MBLs were fabricated, tested and compared. Despite a slightly higher threshold current, the reduced gain-saturation in MBLs results in higher output power. The MBLs also produced a single spatial mode, as well as a strongly dominating single spectral mode which is the inherent feature of MBL-type cavity.

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%.

Super-Homogeneous Saturation of Microwave-Photonic Gain in Optoelectronic Oscillator Systems

We show that the saturation characteristic of microwave-photonic gain is “super-homogeneous” such that the gain of a weaker tone saturates more rapidly than that of a stronger tone when both signals are transmitted over an intensity-modulated optical link. Using this gain model, we simulate the effect of nonlinear gain saturation on the performance of a slab-coupled optical waveguide (SCOW)-based optoelectronic oscillator (OEO). We verify our simulations with experimental measurements and show that low sidemode levels (<; -110 dBc) can be achieved even when multiple modes can oscillate within the passband of the OEO loop filter.

Amplifier-free slab-coupled optical waveguide optoelectronic oscillator systems

We demonstrate a free-running 3-GHz slab-coupled optical waveguide (SCOW) optoelectronic oscillator (OEO) with low phase-noise (<-120 dBc/Hz at 1-kHz offset) and ultra-low sidemode spurs. These sidemodes are indistinguishable from noise on a spectrum analyzer measurement (>88 dB down from carrier). The SCOW-OEO uses high-power low-noise SCOW components in a single-loop cavity employing 1.5-km delay. The noise properties of our SCOW external-cavity laser (SCOWECL) and SCOW photodiode (SCOWPD) are characterized and shown to be suitable for generation of high spectral purity microwave tones. Through comparisons made with SCOW-OEO topologies employing amplification, we observe the sidemode levels to be degraded by any amplifiers (optical or RF) introduced within the OEO cavity.

Low-noise RF-amplifier-free slab-coupled optical waveguide coupled optoelectronic oscillators: physics and operation

We demonstrate a 10-GHz RF-amplifier-free slab-coupled optical waveguide coupled optoelectronic oscillator (SCOW-COEO) system operating with low phase-noise (<-115 dBc/Hz at 1 kHz offset) and large sidemode suppression (>70 dB measurement-limited). The optical pulses generated by the SCOW-COEO exhibit 26.8-ps pulse width (post compression) with a corresponding spectral bandwidth of 0.25 nm (1.8X transform-limited). We also investigate the mechanisms that limit the performance of the COEO. Our measurements indicate that degradation in the quality factor (Q) of the optical cavity significantly impacts COEO phase-noise through increases in the optical amplifier relative intensity noise (RIN).

High-Power, Low-Noise 1.5-μm Slab-Coupled Optical Waveguide (SCOW) Emitters: Physics, Devices, and Applications

We review the development of a new class of high-power, edge-emitting, semiconductor optical gain medium based on the slab-coupled optical waveguide (SCOW) concept. We restrict the scope to InP-based devices incorporating either InGaAsP or InGaAlAs quantum-well active regions and operating in the 1.5-μm-wavelength region. Key properties of the SCOW gain medium include large transverse optical mode dimensions (>;5 × 5 μm), ultralow optical confinement factor (Γ ~ 0.25-1%), and small internal loss coefficient (αi ~ 0.5 cm-1). These properties have enabled the realization of 1) packaged Watt-class semiconductor optical amplifiers (SOAs) having low-noise figure (4-5 dB), 2) monolithic passively mode-locked lasers generating 0.25-W average output power, 3) external-cavity fiber-ring actively mode-locked lasers exhibiting residual timing jitter of <;10 fs (1Hz to Nyquist), and 4) single-frequency external-cavity lasers producing 0.37-W output power with Gaussian (Lorentzian) linewidth of 35 kHz (1.75 kHz) and relative intensity noise (RIN) <; -160 dB/Hz from 200 kHz to 10 GHz. We provide an overview the SCOW design principles, describe simulation results that quantify the performance limitations due to confinement factor, linear optical loss mechanisms, and nonlinear two-photon absorption (TPA) loss, and review the SCOW devices that have been demonstrated and applications that these devices are expected to enable.

Limits to Maximum Absorption Length in Waveguide Photodiodes

The maximum photocurrent, power dissipation, and linearity of waveguide photodiodes are limited by the length over which the input optical power is absorbed. This absorption length is determined by the absorption coefficient of the absorbing layer material (αo), the optical confinement factor (Γ), and the excess loss coefficient (αi). In this paper, we analyze the fundamental limits to maximizing the absorption length and demonstrate a new waveguide photodiode structure that approaches these limits. The new structure is referred to as a slab-coupled optical waveguide photodiode (SCOWPD) and is realized in the InGaAsP/InP material system. Assuming 100% coupling efficiency, the SCOWPD has an ultralow optical confinement factor and a low excess loss coefficient, both calculated from measurements, of Γ = 0.069% and αi = 1.65 cm-1, respectively. This results in a 1/e absorption length of 2.1 mm. The SCOWPD exhibits an external responsivity of 0.8 A/W and a maximum photocurrent of 250 mA at a wavelength of 1.55 μm.

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.

Directly modulated narrowband slab-coupled optical waveguide laser

A slab-coupled optical waveguide laser (SCOWL) is simultaneously wavelength stabilised with a fibre Bragg grating and characterised under a narrow pulse and high repetition rate direct modulation scheme that is suitable for efficient pulse position modulation format optical communication systems. The SCOWL exhibits a large, nearly circular, singlemode that couples efficiently to optical fibre. With a pulse width of 35 ns and a repetition rate of 2.5 MHz, a peak fibre-coupled output power of 1.6 W is achieved for a wavelength stabilised 965 nm SCOWL.

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.