Compact Cavity Research Articles

Progress and Plans for a US Laser System for LISA

A highly stable and robust laser system is a key component of the space-based LISA mission architecture. We describe our progress and plans to demonstrate a TRL 5 LISA laser system at Goddard Space Flight Center by 2020. The laser system includes a low-noise oscillator followed by a power fiber amplifier. The oscillator is a low-mass, compact external cavity laser, consisting of a semiconductor laser coupled to an optical cavity, built by the laser vendor Redfern Integrated Optics. The amplifier is a diode-pumped Yb fiber with 2.5 W output, built at Goddard. We show noise and reliability data for the full laser system, and describe our plans to reach TRL 5.

Formation of long nanosecond rectangular pulses in the active RF pulse compression system with a compact resonant cavity

This work presents the results of the study of an active microwave pulse compression system capable of forming the rectangular pulses with duration ∼10-100 ns, while its dimensions are several times smaller than the radiated wave train. Such compression system is based on the compact planar-voluminal resonant cavity constructed in the shape of a meander from waveguide sections and H-plane tees. The resonant cavity sections are parallel and are in the same plane with tees. The energy input element is located in the input end of the first section. The output device designed as an H-plane tee interference switch is connected to the output end of the last section. Each end of remaining sections is connected through a straight arm to an H-tee with a short-circuited quarter-wave second straight arm. The side arm of each tee is connected with the side arm of the tee in the next section, thus coupling the sections. The short-circuited arm provides the “open” mode and transmission of the wave from tee to tee. We determined the expressions for wave amplitudes in the components of the meander resonant cavity made of three sections and analyzed the expressions as the functions of the parameters determining the oscillation range and energy distribution in the resonant cavity. Experiments demonstrated that under certain conditions the compressors with such resonant cavity could generate nearly rectangular pulses with duration equal to the time of wave two-way traveling along the resonant cavity, and with the power compatible with that of the wave in the resonant cavity, and the length of the radiated wave train several-fold exceeding the size of the compressor. At pulse duration equal to 25 ns, the gain coefficient was 13 dB and pulse power was 40 MW. The work demonstrates the possibility to change the geometry of the resonant cavity by rearranging its components without changing the output pulse parameters.

Simultaneous Measurement of Strain and Temperature Based on Radio-Frequency Demodulation

We propose and experimentally demonstrate a hybrid multimode fiber laser configuration for simultaneous measurement of strain and temperature based on radio-frequency demodulation. The hybrid lasers include a Brillouin erbium fiber laser (BEFL) with dual Brillouin peaks and an erbium-doped fiber laser (EDFL). The BEFL and EDFL are incorporated in a simple and compact ring laser cavity. The injection of Brillouin pump (BP) excites first-order stimulated Brillouin scattering in two sections of fibers with different Brillouin Stokes frequencies. Without BP, the laser performs as an EDFL. The simultaneous measurement of strain and temperature are achieved by monitoring the self-beating signals of the BEFL and EDFL, respectively. The longitudinal modes beat frequency originated from the EDFL and difference of Brillouin frequency shifts from the BEFL are in radio-frequency domain and detected with an electrical spectrum analyzer which makes the detection simplified. The proposed sensor is found to be characterized with simplicity and compact configuration. The maximum measurement errors of strain and temperature obtained are within $\pm 20.3~\mu \varepsilon $ and ±0.5 °C.

Autonomous frequency stabilization of two extended-cavity diode lasers at the potassium wavelength on a sounding rocket

We have developed, assembled, and flight-proven a stable, compact, and autonomous extended cavity diode laser (ECDL) system designed for atomic physics experiments in space. To that end, two micro-integrated ECDLs at 766.7 nm were frequency stabilized during a sounding rocket flight by means of frequency modulation spectroscopy (FMS) of 39^K and offset locking techniques based on the beat note of the two ECDLs. The frequency stabilization as well as additional hard- and software to test hot redundancy mechanisms were implemented as part of a state-machine, which controlled the experiment completely autonomously throughout the entire flight mission.

Submicrosecond Q-Switching Er-Doped All-Fiber Ring Laser Based on Black Phosphorus

Black phosphorus (BP), a new two-dimensional (2D) material, has been deeply developed for extensive applications in electronics and optoelectronics due to its similar physical structure to graphene and thickness dependent direct band gap. Here, we demonstrated a submicrosecond passive Q-switching Er-doped fiber laser with BP as saturable absorber (SA). The BP saturable absorber was fabricated by mechanical exfoliation method. By taking full advantage of the ultrafast relaxation time of BP-SA and careful design of compact ring cavity, we obtained stable Q-switching pulses output with a shortest duration as narrow as 742 ns. With increasing the pump power, the pulse repetition rate accreted gradually almost linearly from 9.78 to 61.25 kHz, and the pulse duration declined rapidly at lower pump power regime and retained approximate stationary at higher pump power regime from 3.05 to 0.742 μs. The experimental results indicate that BP-SA can be an effective SA for nanosecond Q-switching pulse generation.

Compact Circularly Polarized Microstrip Antenna With Cross-Polarization Suppression at Low-Elevation Angle

This letter presents a method of cross-polarization suppression at low-elevation angle for a compact circularly polarized (CP) microstrip antenna. The compact CP antenna adopts a circular radiation patch with a crossed slot to reduce the dimensions, and a four-feeding-point technique is utilized to excite the patch for CP operation. The height and the diameter of the compact CP antenna are $0.064\, \lambda $ and $0.264\, \lambda $ ( $\lambda $ is the free-space wavelength at 1.2 GHz). At the outside of the CP antenna, a compact meander-line ring cavity (MLRC) structure is introduced, which can reduce the cross polarization from –14.2 to –26.1 dB in horizontal direction and increase 3-dB-axial-ratio (AR) ( ${\text{axial}\; \text{ratio}} \leq {\text{3 dB}}$ ) beamwidth from 128° to 192°. The height and diameter of the cavity are just $0.064\, \lambda $ and $0.448\, \lambda.$ Through the experimental validation, the impedance bandwidth of the proposed antenna for VSWR ≤ 2 is 20.8%, and the CP bandwidth for axial ratio ≤ 3 dB is 8.5%. Because of the MLRC, the cross polarization at low-elevation angle is clearly suppressed and the level at $\theta = \pm 90^\circ $ is not more than –23.2 dB. The 3-dB AR beamwidth reaches 190°. In the operating band, the peak gain of 5.3 dBic is also obtained.

Micro-Integrated External Cavity Diode Laser With 1.4-W Narrowband Emission at 445 nm

A compact external cavity diode laser module with 1.4-W narrowband emission at 445 nm is presented. A commercially available broad-area GaN-based laser diode is used as gain medium and a volume Bragg grating is integrated for wavelength stabilization. The laser module is realized on a conduction cooled package with a footprint of 25 mm $\times $ 25 mm. The spectral width is smaller than 50 pm over the whole operating range. A suppression of the amplified spontaneous emission with more than 50 dB is measured. The laser module is suitable for subsequent nonlinear frequency conversion into the deep ultraviolet spectral range.

Cross-Polarized Dual-Frequency VECSEL at 1.5 μm for Fiber-Based Sensing Applications

We have realized a dual-frequency vertical external cavity surface emitting laser (VECSEL) at 1.5 μm. Laser emission of two orthogonally polarized cavity modes is obtained by inserting a birefringent crystal into the VECSEL cavity. We have examined the influence of the different intracavity elements on the laser emission. It is shown that optimizing the free spectral range and the bandwidth of the intracavity Fabry–Perot etalon is of practical importance to achieve a stable single longitudinal laser emission for each of the two orthogonal polarizations. The optimization of the output power has also been investigated, and it is concluded that up to 50 mW output power can be expected in dual-frequency operation by adjusting the reflectivity of the output coupling mirror of the VECSEL cavity. The influence of different parameters on the stability of the dual-frequency emission has been studied. It is concluded that mechanical vibrations are the main cause of radio-frequency (RF) signal instability in our free-running VECSEL cavity. The design of a compact or mono-block cavity may allow meeting the stability requirements for optical fiber sensors based on Brillouin scattering.

Characterization and Crystal Structure of a Robust Cyclohexanone Monooxygenase

Cyclohexanone monooxygenase (CHMO) is a promising biocatalyst for industrial reactions owing to its broad substrate spectrum and excellent regio‐, chemo‐, and enantioselectivity. However, the low stability of many Baeyer–Villiger monooxygenases is an obstacle for their exploitation in industry. Characterization and crystal structure determination of a robust CHMO from Thermocrispum municipale is reported. The enzyme efficiently converts a variety of aliphatic, aromatic, and cyclic ketones, as well as prochiral sulfides. A compact substrate‐binding cavity explains its preference for small rather than bulky substrates. Small‐scale conversions with either purified enzyme or whole cells demonstrated the remarkable properties of this newly discovered CHMO. The exceptional solvent tolerance and thermostability make the enzyme very attractive for biotechnology.

Acousto-optically Q-switched 0.91 μm Nd3+-doped vanadate lasers and 0.45 μm blue-light emissions by intracavity frequency-doubling

Diode-laser pumped Q-switched Nd3+-doped vanadate lasers at 0.91μm and intracavity frequency-doubling of 0.45μm blue light emissions are studied. Using a compact V-type laser cavity and a miniature acousto-optically (AO) Q-switch, the highest average output powers of 2.2W 0.912μm Nd:GdVO4 laser and 1.59W 0.914μm Nd:YVO4 laser are obtained at repetition rate of 10kHz, with the optical conversion efficiency of 7.4% and 5.3%, and the slope efficiency of 14.8% and 9.5%. Employing a BiBO crystal as frequency-doubler, 1.3W, 37ns 0.456μm and 0.84W, 37.4ns 0.457μm pulsed blue light are emitted at 10kHz, corresponding to the peak power of 3.51kW and 2.25kW, respectively. In addition, stable operation is up to 100kHz.

Efficient self-stimulated Raman scattering with simultaneously self-mode-locking in a diode-pumped Nd:GdVO4 laser.

We demonstrated an efficient diode-pumped picosecond self-Raman Nd:GdVO4 laser with the simultaneous processes of stimulated Raman scattering and self-mode-locking in the same crystal. The design of self-mode-locked was theoretically analyzed, and a compact and feasible dual-concave cavity was adopted. The maximum output power of the first-Stokes Raman laser was 736 mW with the repetition rate of 1.51 GHz. In addition, the second-harmonic generation of a yellow laser at 586.5 nm is accomplished with an external LiB3O5 crystal.

The characteristics of Kerr-lens mode-locked self-Raman Nd:YVO4 1176 nm laser

In this paper we report on a compact and feasible dual-concave cavity CW Kerr-lens mode-locked self-Raman Nd:YVO4 laser. A self-starting diode-pumped picosecond Nd:YVO4 1176nm laser is demonstrated without any additional components, where the stimulated Stokes Raman scattering and Kerr-lens-induced mode locking are operated in the same crystal. With an incident pump power of 12W, the average output power at 1176nm is up to 643mW. Meanwhile, the repetition rate and the pulse width of the fundamental laser are measured to be 1.53GHz and 8.6ps, respectively. In addition, the yellow laser output at 588nm is realized by frequency doubling with a LiB3O5 crystal.

Extraordinary Optical Transmission in a Hybrid Plasmonic Waveguide

Extraordinary optical transmission in free space through nanohole arrays has been widely investigated over the past two decades. However, because of its intrinsic nature of periodicity, it suffers from poor integration with nano-/micro- systems. Here, by folding a periodic grating into a compact cavity, we propose a waveguide-based structure within subwavelength, which also exhibits extraordinary optical transmission. Thus, lots of promising applications realized in metallic films can now be migrated onto integrated waveguides in an ultracompact way. Comparing the characteristics of the proposed integrated device and the conventional periodic structure, we reveal the nature of the unexpected transmission spectrum. Based on this principle, a collinear broadband (∼500 nm) polarizer with extinction ratio of >17.8 dB is proposed, with an ultrashort length of 450 nm. Furthermore, this structure has the potential in biological applications when integrated into lab-on-a-chip or microfluidic system. As an example, a biosensor with enhanced sensitivity of 775 nm/RIU is presented.

All-fiber mode-locked laser via short single-wall carbon nanotubes interacting with evanescent wave in photonic crystal fiber.

We report an all-fiber passively mode-locked laser based on a saturable absorber fabricated by filling short single-wall carbon nanotubes into cladding holes of grapefruit-type photonic crystal fiber. The single-wall carbon nanotube is insensitive to polarization of light for its one-dimensional structure, which suppresses the polarization dependence loss. Carbon nanotubes interact with photonic crystal fiber with ultra-weak evanescent field, which enhances the damage threshold of the saturable absorber and improves the operating stability. In our experiment, conventional soliton with a pulse duration of 1.003 ps and center wavelength of 1566.36 nm under a pump power of 240 mW is generated in a compact erbium-doped fiber laser cavity with net anomalous dispersion of -0.4102 ps2. The signal to noise ratio of the fundamental frequency component is ~80 dB. The maximum average output power of the mode-locked laser reaches 9.56 mW under a pump power of 360 mW. The output power can be further improved by a higher pump power.

Compact microdisk cavity laser with GaInNAs/GaAs quantum well

Optically pumped 3-6μm in diameters microdisk lasers with InGaAsN/GaAs quantum well active region has been studied. Single-mode CW lasing at 78K temperature in microdisk laser with 3 μm diameter is demonstrated.

Hybrid III-V/SOI resonant cavity enhanced photodetector.

A hybrid III-V/SOI resonant-cavity-enhanced photodetector (RCE-PD) structure comprising a high-contrast grating (HCG) reflector, a hybrid grating (HG) reflector, and an air cavity between them, has been proposed and investigated. In the proposed structure, a light absorbing material is integrated as part of the HG reflector, enabling a very compact vertical cavity. Numerical investigations show that a quantum efficiency close to 100 % and a detection linewidth of about 1 nm can be achieved, which are desirable for wavelength division multiplexing applications. Based on these results, a hybrid RCE-PD sample has been fabricated by heterogeneously integrating an InP-based material onto a silicon-on-insulator wafer and has been characterized, which shows a clear enhancement in photo-current at the designed wavelength. This indicates that the HG reflector provides a field enhancement sufficient for RCE-PD operation. In addition, a capability of feasibly selecting the detection wavelength during fabrication as well as a possibility of realizing silicon-integrated bidirectional transceivers are discussed.

Single- and dual-wavelength lasers of diode-pumped Nd:LuYAG mixed crystal on various 4F3/2 → 4I13/2 Stark-level transitions

A diode-end-pumped Nd:LuYAG mixed crystal laser in the 1.3 μm spectral domain using a simple and compact two-mirror laser cavity is reported for the first time to our knowledge. In the free-running laser operation, we obtain a simultaneous dual-wavelength laser at about 1319 and 1338 nm with a maximum output power of 2.92 W and a slope efficiency of about 26.7%. By inserting an undoped YAG etalon into the laser cavity, the two 1319 and 1338 nm lasers can be operated singly with the maximum output powers of 1.36 and 1.73 W. Moreover, three simultaneous dual-wavelength lasers at 1334.67 and 1340.93 nm, 1319.15 and 1354.57 nm, and 1335.54 and 1354.94 nm can also be yielded with maximum output powers of 1.02, 0.66, and 0.47 W as well as slope efficiencies of 12.3%, 8.9%, and 6.9%, respectively. These dual-wavelength laser sources provide potential THz wave generation by beat frequency.

Sub-300 femtosecond soliton tunable fiber laser with all-anomalous dispersion passively mode locked by black phosphorus.

By using evanescent field optical deposition method, we had successfully fabricated an effective optoelectronic device based on multi-layer black phosphorus (BP), which is been heavily investigating 2 dimensional (2D) semiconducting material with similar structure as graphene and thickness dependent direct band-gap. By placing this BP-based optoelectronic device inside a highly compact all-anomalous dispersion fiber laser cavity, stable passive mode-locking operation could be ensured and eventually a record 280 fs transmission limited soliton pulse with tunable central wavelength had been obtained through finely tailoring the cavity length. Other operation states, like bound soliton and noise-like state, had also been observed as well. This work demonstrates the enormous potential of BP for ultra-short pulse generation as an effective optoelectronic device.

Single half-effective-wavelength-thick dielectric layer

A compact wideband resonant cavity antenna is presented. A single layer of an approximately half-effective-wavelength-thick partially reflecting surface, which has a very wide transmission band and the positive slope of a reflection phase against frequency, is employed to achieve very wide half power gain bandwidth. The experiment results demonstrate that the antenna has a −10 dB impedance bandwidth of 49.1% (6–9.9 GHz) and a 3 dB gain bandwidth of 48.7% (5.9–9.7 GHz) with a maximum broadside gain of 14.9 dBi. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:1693–1696, 2016

28 μm passively Q-switched Er:CaF_2 diode-pumped laser

A mid-infrared Er doped CaF2 crystal was successfully grown by the bridgeman method. Efficiently continuous wave and Q-switched laser operations were demonstrated at 2.8 μm with a 4 at.% Er doped CaF2 crystal end-pumped by a fiber-coupled 974 nm diode laser. The continuous wave output power of 295 mW was obtained in a compact linear cavity. A stable 2.8 μm passively Q-switched Er:CaF2 laser was also demonstrated with a graphene saturable absorber. Under an absorbed pump power of 2.353 W, an average output power of 172 mW was generated with a pulse duration of 1.324 μs and a repetition rate of 62.70 kHz, corresponding to the single pulse energy of 2.74 μJ and the peak power of 2.07 W, respectively. The high-quality Er:CaF2 crystal and the monolayer graphene are an ideal combination to directly obtain a near 3 μm mid-infrared region pulse laser.