Ho ng Suh

Absolute frequency measurement of an acetylene stabilized laser using a selected single mode from a femtosecond fiber laser comb

We performed an absolute frequency measurement of an acetylene stabilized laser utilizing a femtosecond injection locking technique that can select one component among the fiber laser comb modes. The injection locking scheme has all the fiber configurations. Femtosecond comb lines of 250 MHz spacing based on the fiber femtosecond laser were used for injection locking of a distributed feedback (DFB) laser operating at 1542 nm as a frequency reference. The comb injected DFB laser serves as a selection filter of optical comb modes and an amplifier for amplification of the selected mode. The DFB laser injection locked to the desired comb mode was used to evaluate the frequency stability and absolute frequency measurement of an acetylene stabilized laser. The frequency stability of the acetylene stabilized laser was measured to be 1.1 x 10(-12) for a 1 s averaging time, improving to 6.9 x 10(-14) after 512 s. The absolute frequency of the laser stabilized on the P(16) transition of (13)C(2)H(2) was measured to be 194 369 569 385.7 kHz.

Stable single-frequency fiber ring laser for 25-GHz ITU-T grids utilizing saturable absorber filter

A stable single-frequency fiber ring laser is proposed that operates in a single mode for more than an hour by incorporating unpumped erbium-doped fiber (EDF) as a saturable absorber filter and optimizing the length of EDF used as gain medium. This laser can be continuously tuned to 25-GHz spacing that precisely matches the ITU-T grids by temperature control of etalon filter. This laser had a signal-to-source spontaneous emission ratio higher than 70 dB, and lasing frequencies of 361 channels was matched to ITU-T grids with excellent flatness. Frequency offset from the ITU-T grid was less than 0.14 GHz. The linewidth and the relative intensity noise value was less than 1.3 kHz and 130 dB/Hz (above 250 kHz), respectively.

Optical frequency comb generator based on actively mode-locked fiber ring laser using an acousto-optic modulator with injection-seeding

We present an optical frequency comb generator (OFCG) based on a fiber ring laser that provides a bandwidth of over 1.8 THz with mode-locked pulse operation by external injection seeding. The OFCG was developed via a configuration with actively mode-locked fiber ring laser utilizing an acousto-optic modulator (AOM) with a carrier frequency of 150 MHz when an external seeding laser was injected into the fiber ring cavity. To our knowledge, our experimental device has the widest comb bandwidth ever reported for an OFCG based on a fiber ring loop adopting an AOM device, and ours is the first device that can operate with an actively mode-locked scheme.

Frequency and power stabilization of a three longitudinal mode He-Ne laser using secondary beat frequency

A method of frequency and power stabilization for an internal‐mirror He‐Ne laser using the secondary beat frequency between the three longitudinal modes is presented. Frequency and power fluctuation was ±1 MHz (2 parts in 109) and ±0.10% over 10 000 s, respectively. The square‐root Allan variance between the laser stabilized in this study and the iodine stabilized He‐Ne laser was 5×10−11 at average time of τ=1 s. Furthermore, we could obtain the output power of 2 mW in a single longitudinal mode.

Measurement of the absolute energy level and hyperfine structure of the 87Rb 4D5/2 state.

We have measured the absolute frequency of the excited state transition 5P(3/2)-4D(5/2) in a (87)Rb atom with a femtosecond frequency comb, utilizing the recently developed spectroscopic technique of the double resonance optical pumping method. The absolute energy level of the 4D(5/2) state is determined by measuring the absolute frequency of the 5S(1/2)-5P(3/2) transition simultaneously. The hyperfine structure constants of the 4D(5/2) state are obtained by using the measured frequency. The magnetic dipole constant, A, is determined to be (-16.747+/-0.010) MHz with an uncertainty reduced 60-fold compared with a previous result. The electric quadrupole constant, B, is determined, for what is to our knowledge the first time, to be (4.149+/-0.059) MHz.

Direct frequency counting with enhanced beat signal-to-noise ratio for absolute frequency measurement of a He-Ne/I2 laser at 633 nm.

We obtain a heterodyne beat signal between an iodine-stabilized He-Ne laser at 633 nm and a low power comb mode of an optical frequency synthesizer with an enhanced signal-to-noise ratio (SNR) up to approximately 35 dB. This is accomplished by the adoption of a fiber coupler and the reduction of the shot noise induced by adjacent comb modes. SNR is improved more than 15 dB compared with that of conventional methods. We measure the absolute frequency of the He-Ne laser mentioned above directly utilizing the enhanced SNR. A quantitative analysis of SNR and some information on experimental criteria for the correct frequency counting are given.

Precision spectroscopy of Rb atoms using single comb-line selected from fiber optical frequency comb

We demonstrated the selection of a single comb-line from an optical frequency comb (OFC) of a mode-locked femtosecond fiber laser with a 250 MHz pulse repetition rate, and applied for precision spectroscopy of Rb atoms at 1529 nm. The single comb-line was selected from the fiber-OFC with a 1.5 GHz mode-spacing using spectral-mode-filtering and femtosecond laser injection-locking. When the repetition rate of the mode-locked femtosecond fiber laser was scanned over the range of 382.6 Hz at 250 MHz, we observed the double-resonance optical pumping spectra of the 5S(1/2)-5P(3/2)-4D(3/2) transition of Rb atoms using the selected comb-line of an OFC scanned over the range of 300 MHz at 196,037,213.8 MHz.

A discretely tunable multifrequency source injection locked to a spectral-mode-filtered fiber laser comb

We present a discretely tunable multifrequency source injection locked to an optical frequency comb (OFC) based on the spectral-mode-filtered femtosecond fiber laser. The spectral-mode-filtered OFC with a 1.5 GHz mode spacing, which is based on an femtosecond fiber laser (FSFL) with 250 MHz mode spacing, was achieved using the spectral-mode-filtering method with a Fabry–Perot cavity. With the spectral-mode-filtered OFC as the master laser and two distributed-feedback lasers as the slave lasers, we simultaneously selected and amplified the desired modes of the OFC using the IL technique. We generated the coherent multifrequency optical source to synthesize in the frequency range from gigahertz to terahertz.

Widely Tunable External Cavity Laser Diode Injection Locked to an Optical Frequency Comb

We report an ultrastable external cavity laser diode (ECLD) that can be selectively injection locked to an optical frequency comb (OFC) generated by acetylene stabilized laser (ASL) seeding. The carrier envelope offset (position) and repetition rate (mode spacing) of the OFC, which functions as the master oscillator, was stabilized to the ASL (1.1 × 10-12 at 1 s) and H-maser (2 × 10-13 at 1 s), respectively. The ECLD can be tuned over 70 nm in the 1.5-μm region and can be discretely locked to the comb modes with 25-GHz spacing over a 10-THz span. The frequency stability of the ECLD injection locked to the OFC reached as high as 1.1 × 10-12 for 1-s averaging time.

Discretely Tunable Optical Frequency Synthesizer Utilizing a Femtosecond Fiber Laser Injection-Locking Technique

In this letter, we propose a discretely tunable optical frequency synthesizer (DTOFS) based on distributed-feedback (DFB) lasers that can be selectively injection locked to a desired single mode of the femtosecond fiber laser comb. The methods for the injection locking, the monitoring of the lock status, and the selection of the desired comb mode are given. We demonstrate the discrete tunability of the DTOFS by constructing two frequency-stabilized DFB lasers, both of which are injection locked to a respective single mode of the fiber laser comb with the frequency difference, which is an exact multiple of the comb repetition frequency.