Main Amplifier Research Articles

520 μJ Microsecond Burst-Mode Pulse Fiber Amplifier with GHz-Tunable Intra-Burst Pulse and Flat-Top Envelope

We present a 520 μJ microsecond burst-mode pulse fiber amplifier with a GHz-tunable intra-burst repetition rate and a nearly flat-top pulse envelope. The amplifier architecture comprises a microsecond pulse seed, a high-bandwidth electro-optic modulator (EOM), two pre-amplifier stages, a waveform-compensated acoustic-optic modulator (AOM), and two main amplifier stages. To address amplified spontaneous emission (ASE) and nonlinear effects, a multistage synchronous pumping scheme that achieved a maximum energy output of 520 μJ and has a peak power of 160 W was used. To produce a flat-topped burst pulse envelope, the AOM generates an editable waveform with a leading edge and a high trailing edge to compensate for waveform distortion, resulting in a 5 μs nearly flat-top pulse envelope at maximum energy. The laser provides an adjustable intra-burst pulse repetition rate range of 1–5 GHz through the high-bandwidth EOM modulation. The intra-burst pulse jitter time of the laser remains below 4.31 ps at different frequencies. Moreover, the beam quality of the amplifier is M2x = 1.04 and M2y = 1.1. This amplifier exhibits promising potential and can be further amplified as an optical drive source for high-power, large-bandwidth microwave photon (MWP) radar applications. Meanwhile, it is also potentially applicable as a pulse source for high-speed optical communications, the high-precision processing of special materials, and LIDAR ranging.

A new design method for distributed Doherty power amplifier

This paper introduces the basic theory of a new type of Distributed Doherty Power Amplifier (DDPA) architecture, which uses a step-by-step current injection method for load modulation to improve the modulation network order of the main amplifier and reduce the impedance conversion ratio, thereby increasing the operating bandwidth. The load modulation trajectory and drain efficiency under single-frequency point and wideband range with 6 dB and 8 dB back-off were analyzed, and the effect of peaking branches output phase on the load modulation trajectory and drain efficiency curve were also analyzed. Finally, a DDPA working at 1.5–2.7 GHz (57.1 % relative bandwidth) was designed using Wolfspeed’s CGH40025 transistor. The physical simulation results show that its saturated output power reaches 47.7–48.8 dBm, the saturated gain is 5.17–8.9 dB, and the saturated drain efficiency is 46.53–61 %. The drain efficiency under 8 dB back-off is 43.5–53.7 %, and the drain efficiency under 6 dB back-off is 58.37 %-67.85 %.

414.9 W in-band pumped Er/Yb co-doped fiber amplifier seeded by a random fiber laser.

We demonstrated a 414.9 W large-mode-area Er/Yb co-doped fiber amplifier with a good time-domain stability, seeded by a 1568 nm random fiber laser (RFL) seed with a half-open cavity. We believe this to be the highest output power of the RFL in a 1.5 µm band to date. At the maximum output power, the optical efficiency was 36.9% and the 3 dB linewidth was 0.69 nm. The 1535 nm fiber lasers were used as the pumping source of the main amplifier to avoid the parasitic lasing of Yb ions. The spectral linewidth broadening was suppressed during the amplification process due to the stable temporal output of the RFL seed. The M2 factor increased from 2.28 at 145.7 W to 2.83 at 280.8 W. Our research provides a robust approach for achieving high-power and high-spectral-purity RFLs in the 1.5 µm band.

A CMOS Optoelectronic Transimpedance Amplifier Using Concurrent Automatic Gain Control for LiDAR Sensors

This paper presents a novel optoelectronic transimpedance amplifier (OTA) for short-range LiDAR sensors used in 180 nm CMOS technology, which consists of a main transimpedance amplifier (m-TIA) with an on-chip P+/N-well/Deep N-well avalanche photodiode (P+/NW/DNW APD) and a replica TIA with another on-chip APD, not only to acquire circuit symmetry but to also obtain concurrent automatic gain control (AGC) function within a narrow single pulse-width duration. In particular, for concurrent AGC operations, 3-bit PMOS switches with series resistors are added in parallel with the passive feedback resistor in the m-TIA. Then, the PMOS switches can be turned on or off in accordance with the DC output voltage amplitudes of the replica TIA. The post-layout simulations reveal that the OTA extends the dynamic range up to 74.8 dB (i.e., 1 µApp~5.5 mApp) and achieves a 67 dBΩ transimpedance gain, an 830 MHz bandwidth, a 16 pA/√Hz noise current spectral density, a −31 dBm optical sensitivity for a 10−12 bit error rate, and a 6 mW power dissipation from a single 1.8 V supply. The chip occupies a core area of 200 × 120 µm2.

Polarization extinction ratio promotion in high-power linearly polarized fiber lasers

This article establishes a model for analyzing polarization extinction ratio (PER) characteristics of high-power linearly polarized fiber lasers. By combining thermal-induced polarization coupling with mode coupling, the distribution characteristics of PER along the fiber are calculated, and the main physical factors affecting PER are summarized. The model is employed to analyze the correlations between PER and the fiber length, bending radius, as well as pumping scheme of the main amplifier in the linearly polarized fiber laser. Subsequently, the experimental testing examined the impact of coiling shape on PER. Finally, a high-power linearly polarized fiber laser based on an oscillator is constructed using optimized parameters, producing an output power of 3 kW and a PER of 20.8 dB.

Kilowatt-Level High-Efficiency Narrow-Linewidth All-Fiber Tm3+-Doped Laser

In this study, a kilowatt-level high-efficiency narrow-linewidth all-fiber Tm3+-doped continuous-wave laser operating at 1.95 μm is demonstrated. Benefitting from an advanced boost design of a two-stage main amplifier, it not only effectively manages heat dissipation resulting from the high pump-induced quantum defect but also realizes the controlled extraction of optical gain and improves the optical conversion efficiency. Finally, this laser system has realized an output power of 1018 W, a linewidth of 3.8 GHz, and a slope efficiency of 60.0% simultaneously. Moreover, a high optical signal-to-noise ratio of over 45 dB and excellent beam quality of M2 factors 1.19 are obtained. To the best of our knowledge, this represents the narrowest linewidth and highest slope efficiency achieved in a kilowatt-level Tm³⁺-doped fiber laser. Such a high-performance laser is ideally suited for mid-infrared generation and remote sensing applications.

Passively harmonic mode-locked GHz U-band femtosecond pulse generation based on soliton self-frequency shift.

We demonstrate a GHz U-band fiber laser harnessing soliton self-frequency shift (SSFS). The seed source is a passively harmonic mode-locked (HML) fiber laser based on carbon nanotubes (CNTs) polymer film. By adjusting the pump power and polarization controller (PC) appropriately, the repetition rate can be tuned up to 1.104 GHz with a side mode suppression ratio (SMSR) of 42.5 dB. The seed source with different repetition rates can be frequency shifted from C-band to U-band in a piece of dispersion-shifted fiber (DSF) after amplification. We selected three repetition rates (457.8 MHz, 700.1 MHz, and 1104.1 MHz) and systematically studied the impact of different repetition rates on SSFS. The broad tuning range of wavelengths covering the entire U-band can be obtained by adjusting the pump power of the main amplifier. To the best of our knowledge, the demonstrated laser is the first U-band femtosecond fiber laser based on passively HML with a repetition rate over 1 GHz.

A 0.064 mm2 16-Channel In-Pixel Neural Front End with Improved System Common-Mode Rejection Exploiting a Current-Mode Summing Approach

In this work, we introduce the design of a 16-channel in-pixel neural analog front end that employs a current-based summing approach to establish a common-mode feedback loop. The primary aim of this novel structure is to enhance both the system common-mode rejection ratio (SCMRR) and the common-mode interference (CMI) range. Compared to more conventional designs, the proposed front end utilizes DC-coupled inverter-based main amplifiers, which significantly reduce the occupied on-chip area. Additionally, the current-based implementation of the CMFB loop obviates the need for voltage buffers, replacing them with simple common-gate transistors, which, in turn, decreases both area occupancy and power consumption. The proposed architecture is further examined from an analytical standpoint, providing a comprehensive evaluation through design equations of its performance in terms of gain, common-mode rejection, and noise power. A 50 μm × 65 μm compact layout of the pixel amplifiers that make up the recording channels of the front end was designed using a 180 nm CMOS process. Simulations conducted in Cadence Virtuoso reveal an SCMRR of 80.5 dB and a PSRR of 72.58 dB, with a differential gain of 44 dB and a bandwidth that fully encompasses the frequency range of the bio-signals that can be theoretically captured by the neural probe. The noise integrated in the range between 1 Hz and 7.5 kHz results in an input-referred noise (IRN) of 4.04 μVrms. Power consumption is also tested, with a measured value of 3.77 μW per channel, corresponding to an overall consumption of about 60 μW. To test its robustness with respect to PVT and mismatch variations, the front end is evaluated through extensive parametric simulations and Monte Carlo simulations, revealing favorable results.

Analysis and design of a GHz bandwidth adaptive bias circuit for an mmW Doherty amplifier

This paper derives theoretical results for adaptive bias in Doherty amplifiers and presents the design and measurements of an integrated adaptive bias circuit tailored for high peak-to-average high bandwidth signals. Fundamental equations for output power, impedance, and efficiency of the complete Doherty amplifier are derived. Even with ideal transistor models, the Doherty amplifier is fundamentally nonlinear due to saturation of the main amplifier and class-C nonlinearity of the auxiliary. Increasing the transconductance of the auxiliary amplifier mitigates the distortion. Adaptive bias offers the possibility to control the output current characteristic of the auxiliary amplifier. This means that adaptive bias linearises and mitigates the need for an oversized auxiliary amplifier. Both methods, transconductance scaling and adaptive bias, are analysed and compared as well as having a band limited adaptive bias signal. The design of a multiple GHz bandwidth adaptive bias circuit is presented. To verify the circuit design and the theoretical predictions, an mmW Doherty amplifier in 22 nm CMOS-FD-SOI, utilizing the presented adaptive bias circuit, is measured and compared with and without adaptive bias. Comparison is conducted both using continuous-wave and modulated high bandwidth signals. Measured results confirm the predicted improvements by the adaptive bias as derived by the theoretical analysis.

5.4 W, 2.35 µm cascaded Raman fiber laser pumped by dissipative soliton resonance-like pulses

We present a nonlinear amplifying loop mirror-based mode-locked fiber laser. By adjusting the pump power, the proposed laser exhibits a dissipative soliton resonance (DSR)-like pulse operation with a maximum pulse width of 150 ns. Subsequently, a three-stage Tm3+-doped fiber amplifier is implemented using a single-mode double-cladding Tm3+-doped fiber to increase the DSR-like pulse output power to 52.5 W, achieving a pump slope efficiency of 47.1% in the main amplifier. A 25 m first-order Raman-gain fiber (UHNA7) is pumped by a DSR-like pulse, and 16.3 W of pure 2.135 µm first-order Raman light with a spectral purity of 73.4% is obtained. Finally, 5.4 W of 2.35 µm second-order Raman light with a spectral purity of 66% is obtained using a 10 m 98% germania-core fiber as a second-order Raman-gain fiber cascaded after UHNA7 fiber. To the best of our knowledge, this is the highest output power ever obtained from a 2.3 µm laser.

A new longitudinal pumping scheme that uses elliptical laser beams to improve the compactness and efficiency of high-power dye lasers

Liquid dye lasers are intrinsically suitable for high-average-power operation because their flowing laser gain media can effectively remove the heat produced by laser amplification; thus, liquid dye lasers have a significant place in various laser applications. However, to sufficiently remove the dye solution heated by the laser, a very cumbersome dye flow system is required to achieve a high flow velocity in the dye cell of a high-power dye laser, which limits its applications. In this paper, we found that the pressure and power required to drive the high-speed dye flow are both proportional to the square of the laser beam size along the direction of the flow. Based on this result, we proposed a novel longitudinal pumping scheme that uses elliptical laser beams. This new scheme can significantly reduce the size and energy consumption of dye flow loops in high-power dye lasers. We also developed a dye laser system with a main amplifier using a 1 mm × 4 mm elliptical input laser beam. The amplifier produced an average power of 135 W with a repetition rate of 10 kHz and a conversion efficiency of 39.5%. The M2 factor of the output beam was better than 1.7. The flow parameters to drive the dye flow loops were 2.1×105 Pa, and 0.43 L/s, which gave a clearing ratio of 2 for the amplifier. This new pumping scheme enables high-power dye lasers to be more compact and efficient, making them more flexible for various applications.

High-Power Raman Soliton Generation at 1.7 μm in All-Fiber Polarization-Maintaining Erbium-Doped Amplifier

An all-fiber polarization maintaining high-power laser system operating at 1.7 μm based on the Raman-induced soliton self-frequency shifting effect is demonstrated. The entirely fiberized system is built by erbium-doped oscillator and two-stage amplifiers with polarization maintaining commercial silica fibers and devices, which can provide robust and stable soliton generation. High-power soliton laser with the average power of 0.28 W, the repetition rate of 42.7 MHz, and pulse duration of 515 fs is generated directly from the main amplifier. Our experiment provides a feasible method for high-power all-fiber polarization maintaining femtosecond laser generation working at 1.7 μm.

3.05 kW, 13.7 GHz linewidth fiber amplifier based on PRBS phase modulation for SBS suppression

In this paper, we establish a multi-stage fiber amplifier with pseudo-random binary sequence (PRBS) phase modulation. The stimulated Brillouin gain spectra of the main amplifier with both the unmodulated and pseudo-random binary sequence phase modulated configuration are measured (with corresponding output power), and the stimulated Brillouin scattering (SBS) threshold is investigated experimentally and theoretically. The pseudo-random binary sequence phase modulation parameters are optimized by theoretical simulation. With a two-stage preamplifier chain and a counter-pumping main amplifier stage, a maximum 3.05 kW output power with a slope efficiency of 85.9% is obtained experimentally. The central wavelength of the fiber amplifier is 1050 nm, associated with a full-width at half-maximum linewidth of 13.7 GHz. The stimulated Brillouin scattering reflectivity is below 0.01% at 3.05 kW at 13.7 GHz, which indicates that stimulated Brillouin scattering can be suppressed efficiently at this power and linewidth level.

An ultrahigh input impedance low noise analog front‐end design for epilepsy brain recording system

SummaryThis paper presents an ultrahigh input impedance, low noise, and wide bandwidth four‐channels analog front‐end (AFE) for low‐power neural recording systems. To achieve high input impedance, the buffer channels are placed between the electrodes and the main amplifier stage of the AFE. The buffer is designed with low noise and low power consumption to obtain high input impedance of overall AFE while maintaining low input‐referred noise and low power consumption. A chopper capacitively coupled chopper instrumentation amplifier (CCIA) is placed after the buffer as the main amplifier stage of the AFE to improve the common mode rejection ratio (CMRR) and the input‐referred noise of the overall AFE design. A new chopper stabilization control technique is proposed and used in the CCIA stage to reduce the charge injection and clock feedthrough and consequently the high‐frequency ripple of the AFE output signal. A programmable gain amplifier (PGA) is designed as the third stage to adjust the overall gain of the AFE. Benefiting from PGA, the AFE can adapt its gain with both action potential and local field potential signals. To reduce the number of the electrode to be implanted and reduce the impedance mismatch that causes the degradation on overall CMRR performance, two AFE channels shared a reference electrode followed by a reference buffer. The proposed AFE is designed and simulated using a standard 180 nm CMOS process and operates in a wide frequency band of 2.1 to 2500 Hz with low input‐referred noise of 1.6 μVrms and a CMRR over 80 dB at 2.1 Hz. The total power consumption is lower than 4.3 μW per channel. With the proposed structure of AFE, the input impedance is 35 GΩ @ 21 Hz and the minimum impedance over operational bandwidth is 75 MΩ @ 2.5 kHz.

Robust low-noise 795-nm single-frequency fiber laser with continuous frequency tuning

This study presents a highly efficient and low-noise all-fiber single-frequency (SF) 795-nm laser system realized using a simple and robust single-pass frequency-doubling configuration. The foundation of the system is a 1590-nm erbium-doped fiber amplifier operating in a master oscillator power amplifier configuration seeded by a distributed feedback semiconductor laser at 1590.52 nm. The main amplifier exhibits an exceptional amplification efficiency of 30.5 %. By utilizing a periodically poled lithium niobate waveguide, the 795-nm laser is frequency-doubled with an output power reaching 2.3 W, corresponding to an optical efficiency of 44 % of the fundamental laser. Crucial to its application in precision measurements and quantum technologies, the SF 795-nm laser exhibits a relative intensity noise close to −150 dBc/Hz in the frequency range of 0.1 to 10 MHz. Additionally, a continuous frequency-tuning range of 11.7 GHz is achieved by modulating the current with a 2.5-mA 1-Hz triangular wave. Notably, this is the first report on an all-fiber SF 795-nm laser system.

Ultra-flat and high-efficient mid-infrared supercontinuum generation in indium fluoride fiber

We demonstrate ultra-flat and high-efficient mid-infrared supercontinuum (MIR SC) generation in indium fluoride (InF3) fiber with output average power of 9.9 W, power conversion efficiency of 67.8 % and 3 dB-spectrum width of >1570 nm. It is pumped by amplified 2 μm noise-like pulses and generated in a piece of 10 m InF3 fiber with core diameters of 9.5 μm. The 2 μm noise-like pulses is realized by nonlinear amplifying loop mirror mode-locking technique and amplified by two-stage amplifiers with pulse repetition rate of 1.513 MHz and 3-dB pulse envelope width of 2.2 ns. To further increase efficiency, the InF3 fiber is spliced directly to the pigtail of main amplifier with 92.47 % coupling efficiency, which is in good agreement with 92 % theoretical result of fundamental-mode coupling efficiency. To the best of the authors’ knowledge, this work achieves the flattest (3 dB spectral coverage of 2120–3690 nm) and corresponding highest efficiency (67.8 %) in MIR SC seeded by 2 μm pulses.

A 7-kW narrow-linewidth fiber amplifier assisted by optimizing the refractive index of the large-mode-area active fiber

Abstract The high-power narrow-linewidth fiber laser has become the most widely used high-power laser source nowadays. Further breakthroughs of the output power depend on comprehensive optimization of stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS) and transverse mode instability (TMI). In this work, we aim to further surpass the power record of all-fiberized and narrow-linewidth fiber amplifiers with near-diffraction-limited (NDL) beam quality. SBS is suppressed by white-noise-signal modulation of a single-frequency seed. In particular, the refractive index of the large-mode-area active fiber in the main amplifier is controlled and fabricated, which could simultaneously increase the effective mode field area of the fundamental mode and the loss coefficient of higher-order modes for balancing SRS and TMI. Subsequent experimental measurements demonstrate a 7.03 kW narrow-linewidth fiber laser with a signal-to-noise ratio of 31.4 dB and beam quality factors of Mx2 = 1.26, My2 = 1.25. To the best of our knowledge, this is the highest reported power with NDL beam quality based on a directly laser-diode-pumped and all-fiberized format, especially with narrow-linewidth spectral emission.

A 172 mJ, high-energy picosecond 355 nm ultraviolet laser system at 100 Hz

Abstract A high-energy picosecond 355 nm ultraviolet (UV) laser operating at 100 Hz was demonstrated. A 352 mJ, 69 ps, 1064 nm laser at 100 Hz was realized firstly by cascaded regenerative, laser diode end-pumped single-pass and side-pumped main amplifiers. The stimulated Raman scattering-based beam shaping technique, thermally induced birefringence compensation and 4f spatial filter-image relaying systems were used to maintain a relatively homogeneous beam intensity distribution during the amplification process. By using lithium triborate crystals for second- and third-harmonic generation (THG), a 172 mJ, approximately 56 ps, 355 nm UV laser was achieved with a THG conversion efficiency of 49%. To the best of our knowledge, it is the highest pulse energy of a picosecond 355 nm UV laser so far. The beam quality factor ${M}^2$ and pulse energy stability were ${M}_x^2$ =3.92, ${M}_y^2$ =3.71 and root mean square of 1.48%@3 hours. This laser system could play significant roles in applications including photoconductive switch excitation, laser drilling and laser micro-fabrication.

Development Review of Highly Efficient Sequential Power Amplifier with Extended Back-Off Range for Broadband Application

Similar to a Doherty power amplifier (DPA), a sequential power amplifier (SPA) is mainly composed of a main amplifier, an auxiliary amplifier and a combiner. However, SPA breaks the bandwidth limitation of the impedance inverter in the DPA, and also simplifies the design procedure. Since the main amplifier has no load modulation, it is easy for the SPA to realize broadband operation and improve the output back-off (OBO) power range. Therefore, SPA has great advantages and potential in expanding bandwidth, improving drain efficiency and expanding the back-off range of a power amplifier simultaneously. This paper describes the evolution and classification of the SPA. First, the basic theory of the SPA is reviewed. Then, some two-way SPAs using coupler and circulator as a power combiner are discussed. Thirdly, the latest popular sequential load modulated balanced amplifier is overviewed.

Design of a Configurable 4-Channel Analog Front-End for EEG Signal Acquisition on 180nm CMOS Process

In this work, a 4-channel Analog Front-End (AFE) circuit has been proposed for EEG signal recording. For EEG recording systems, the AFE may handle a wide range of sensor inputs with high input impedance, adjustable gain, low noise, and wide bandwidth. The buffer or current-to-voltage converter block (BCV), which can be set to operate as a buffer or a current-to-voltage converter circuit, is positioned between the electrode and the main amplifier stages of the AFE to achieve high input impedance and work with sensor signal types. A chopper capacitively-coupled instrumentation amplifier (CCIA) is positioned after the BCV as the main amplifier stage of the AFE to reduce input-referred noise and balance the impedance of the overall AFE system. A programmable gain amplifier (PGA) is the third stage of the AFE that allows the overall gain of the AFE to be adjusted. The suggested AFE operates in a wide frequency range of 0.5 Hz to 2 kHz with a high input impedance bigger than 2TΩ, and it is constructed and simulated using a 180nm CMOS process. With the lowest 100-dB CMRR and low input-referred noise of 1.8 µVrms, the AFE can achieve low noise efficiency. EEG signals can be acquired with this AFE system, which is very useful for detecting epilepsy and seizures.