Cascaded half-harmonic generation of femtosecond frequency combs in mid-IR

For the growing demand of frequency combs in mid-infrared (mid-IR), known as the "molecular fingerprint" region of the spectrum [1], down conversion of near-IR frequency combs through half- harmonic generation offers numerous benefits including high conversion efficiency and intrinsic phase and frequency locking to the near-IR pump [2]. Hence cascaded half-harmonic generation promises a simple path towards extending the wavelength coverage of stable frequency combs. Here, we report a two-octave down-conversion of a frequency comb around 1 {\mu}m through cascaded half-harmonic generation with ~64% efficiency in the first stage, and ~18% in the second stage. We obtain broadband intrinsically-frequency-locked frequency combs with ~50-fs pulses at ~2 {\mu}m and ~110-fs pulses at ~4 {\mu}m. These results indicate the effectiveness of half-harmonic generation as a universal tool for efficient phase- and frequency-locked down-conversion, which can be beneficial for numerous applications requiring long-wavelength coherent sources

Evaluation of Beam Quality Study of Arbitrary Beam Profiles from On-Wafer Vertical Cavity Surface Emitting Lasers

Vertical cavity surface emitting lasers (VCSELs) have found mainstream use in data centers and short-haul optical fiber communications. Along with the increase in the capacity of such systems comes an increase in the demand for greater power efficiency. System evaluation now includes an assessment of the energy required for each bit of data, a metric referred to as ‘joules per bit’. One source of loss for VCSELs is coupling loss, which is due to a mismatch in the mode profiles of the VCSELs and the optical fiber into which the VSCEL light is coupled. One way to reduce this loss is to develop single-mode VCSEL devices that are modally matched to optical fiber. Efficient development of these devices requires a technique for rapidly evaluating beam quality. This study investigates the use of a vertically mounted commercial beam profiling system and hardware interface software to quickly evaluate the beam quality of arbitrary beam profiles from on-wafer mounted VCSEL devices. This system captures the beam profile emitted from a VCSEL device at fixed locations along the vertical axis. Each image is evaluated within software along a predetermined axis, and the beam quality, or M2, is calculated according to international standards. This system is quantitatively compared against a commercial software package designed for determining beam quality across a fixed axis

Evaluation of Beam Quality Study of Arbitrary Beam Profiles from On-Wafer Vertical Cavity Surface Emitting Lasers

Vertical cavity surface emitting lasers (VCSELs) have found mainstream use in data centers and short-haul optical fiber communications. Along with the increase in the capacity of such systems comes an increase in the demand for greater power efficiency. System evaluation now includes an assessment of the energy required for each bit of data, a metric referred to as ‘joules per bit’. One source of loss for VCSELs is coupling loss, which is due to a mismatch in the mode profiles of the VCSELs and the optical fiber into which the VSCEL light is coupled. One way to reduce this loss is to develop single-mode VCSEL devices that are modally matched to optical fiber. Efficient development of these devices requires a technique for rapidly evaluating beam quality. This study investigates the use of a vertically mounted commercial beam profiling system and hardware interface software to quickly evaluate the beam quality of arbitrary beam profiles from on-wafer mounted VCSEL devices. This system captures the beam profile emitted from a VCSEL device at fixed locations along the vertical axis. Each image is evaluated within software along a predetermined axis, and the beam quality, or M2, is calculated according to international standards. This system is quantitatively compared against a commercial software package designed for determining beam quality across a fixed axis

Fractional-length sync-pumped degenerate optical parametric oscillator for 500-MHz 3-μm mid-infrared frequency comb generation

We demonstrate a mid-IR frequency comb centered at 3120 nm with 650-nm (20-THz) bandwidth at a comb-teeth spacing of 500 MHz. The generated comb is based on a compact ring-type synchronously pumped optical parametric oscillator (SPOPO) operating at degeneracy and pumped by a mode-locked Er-doped 1560 nm fiber laser at a repetition rate of 100 MHz. We achieve high-repetition rate by using a fractional-length cavity with a roundtrip length of 60 cm, which is one-fifth of the length dictated by conventional synchronous pumping

Temporal Simultons in Optical Parametric Oscillators

We report the first demonstration of a regime of operation in optical parametric oscillators (OPOs), in which the formation of temporal simultons produces stable femtosecond half-harmonic pulses. Simultons are simultaneous bright-dark solitons of a signal field at frequency ω and the pump field at 2ω, which form in a quadratic nonlinear medium. The formation of simultons in an OPO is due to the interplay of nonlinear pulse acceleration with the timing mismatch between the pump repetition period and the cold-cavity round-trip time and is evidenced by sech^2 spectra with broad instantaneous bandwidths when the resonator is detuned to a slightly longer round-trip time than the pump repetition period. We provide a theoretical description of an OPO operating in a regime dominated by these dynamics, observe the distinct features of simulton formation in an experiment, and verify our results with numerical simulations. These results represent a new regime of operation in nonlinear resonators, which can lead to efficient and scalable sources of few-cycle frequency combs at arbitrary wavelengths

Quadratic Soliton Frequency Comb at 4 µm from an OP-GaP-based Optical Parametric Oscillator

We report generation of quadratic solitons, i.e. temporal simultons, in an OP-GaP based halfharmonic optical parametric oscillator. We achieve 4-µm pulses with sech² spectrum of 790nm FWHM bandwidth, 197% slope efficiency, and 38% conversion efficiency

Measuring M2 values for on-wafer vertical cavity surface emitting lasers

We report on M2 measurements taken for on-wafer vertical cavity surface emitting lasers (VCSELs). We measured M2 for oxide-confined VCSELs and photonic crystal (PhC) VCSELs of similar lasing aperture sizes

Sub-50 fs pulses around 2070 nm from a synchronously-pumped, degenerate OPO

We report generation of 48 fs pulses at a center wavelength of 2070 nm using a degenerate optical parametric oscillator (OPO) synchronously-pumped with a commercially available 36-MHz, femtosecond, mode-locked, Yb-doped fiber laser. The spectral bandwidth of the output is ~137 nm, corresponding to a theoretical, transform-limited pulse width of 33 fs. The threshold of the OPO is less than 10 mW of average pump power. By tuning the cavity length, the output spectrum covers a spectral width of more than 400 nm, limited only by the bandwidth of the cavity mirrors

Characterization of Single-Mode Vertical Cavity Surface-Emitting Lasers

A high-quality single-mode beam is desirable for the efficient use of lasers as light sources for optical data communications and interconnects, however there is little data which characterizes operating ranges and near-field beam qualities of Vertical Cavity Surface Emitting Lasers (VCSELs), which has resulted in a lack of analysis of these devices. Measures of beam quality include beam-quality factor (M2 ), Side-Mode-Suppression-Ratio (SMSR) and RMS linewidth. M2 is a measurement of how closely the beam is to an ideal Gaussian. SMSR is the difference, in dB, between the amplitude of the primary peak and the amplitude of the next highest peak of the output spectrum, with single-mode operation defined by a SMSR \u3e 30 dB. RMS linewidth is a second moment calculation involving the power spectral density, where smaller RMS linewidth indicates higher beam quality. Utilizing a novel vertical M2 setup in which on-wafer VCSEL M2 can be measured, a study was conducted on the relation between M2 , SMSR and RMS linewidth, for various oxide-confined VCSELs of varying aperture sizes and Photonic Crystal (PhC) VCSELs of varying aperture sizes and photonic crystal configurations. First, the operating range of the VCSEL was determined utilizing a Semiconductor Parameter Analyzer to obtain the LIV characteristics. Along with this measurement, spectral data was collected using an Optical Spectrum Analyzer at several key operating points, which allowed the RMS linewidths and SMSRs of the devices to be calculated at these points. The novel beam-profiler setup was used to measure the device’s M2 . Initial results show a strong correlation between the measures of beam quality, with increasing SMSR, corresponding to M2 values closer to 1, and single-mode operation characterized by a M2 of less than 1.5. A strong correlation between RMS linewidth and M2 was also seen, with increasing RMS linewidths corresponding to an increase in M2

Characterization of Single-Mode Vertical Cavity Surface-Emitting Lasers

A high-quality single-mode beam is desirable for the efficient use of lasers as light sources for optical data communications and interconnects, however there is little data which characterizes operating ranges and near-field beam qualities of Vertical Cavity Surface Emitting Lasers (VCSELs), which has resulted in a lack of analysis of these devices. Measures of beam quality include beam-quality factor (M2 ), Side-Mode-Suppression-Ratio (SMSR) and RMS linewidth. M2 is a measurement of how closely the beam is to an ideal Gaussian. SMSR is the difference, in dB, between the amplitude of the primary peak and the amplitude of the next highest peak of the output spectrum, with single-mode operation defined by a SMSR \u3e 30 dB. RMS linewidth is a second moment calculation involving the power spectral density, where smaller RMS linewidth indicates higher beam quality. Utilizing a novel vertical M2 setup in which on-wafer VCSEL M2 can be measured, a study was conducted on the relation between M2 , SMSR and RMS linewidth, for various oxide-confined VCSELs of varying aperture sizes and Photonic Crystal (PhC) VCSELs of varying aperture sizes and photonic crystal configurations. First, the operating range of the VCSEL was determined utilizing a Semiconductor Parameter Analyzer to obtain the LIV characteristics. Along with this measurement, spectral data was collected using an Optical Spectrum Analyzer at several key operating points, which allowed the RMS linewidths and SMSRs of the devices to be calculated at these points. The novel beam-profiler setup was used to measure the device’s M2 . Initial results show a strong correlation between the measures of beam quality, with increasing SMSR, corresponding to M2 values closer to 1, and single-mode operation characterized by a M2 of less than 1.5. A strong correlation between RMS linewidth and M2 was also seen, with increasing RMS linewidths corresponding to an increase in M2