Sung Hun Lee

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.

Precision depth measurement of through silicon vias (TSVs) on 3D semiconductor packaging process

We have proposed and demonstrated a novel method to measure depths of through silicon vias (TSVs) at high speed. TSVs are fine and deep holes fabricated in silicon wafers for 3D semiconductors; they are used for electrical connections between vertically stacked wafers. Because the high-aspect ratio hole of the TSV makes it difficult for light to reach the bottom surface, conventional optical methods using visible lights cannot determine the depth value. By adopting an optical comb of a femtosecond pulse laser in the infra-red range as a light source, the depths of TSVs having aspect ratio of about 7 were measured. This measurement was done at high speed based on spectral resolved interferometry. The proposed method is expected to be an alternative method for depth inspection of TSVs.

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.

Ultra-stable multi-frequency source injection locked to a spectral-mode-filtered fiber laser comb for terahertz generation

We present a discretely tunable multi-frequency source injection locked to an optical frequency comb (OFC) based on the spectral-mode-filtered femtosecond fiber laser (FSFL). The spectral-mode-filtered OFC (SMFD-OFC) with 1.5 GHz mode spacing was achieved using the spectral-mode-filtering (SMFG) method with a Fabry-Pérot cavity. With the SMFD-OFC as the master laser and two distributed-feedback (DFB) lasers as the slave lasers, we simultaneously selected and amplified the desired modes of the OFC using the injection locking (IL) technique. We then generated a widely separated coherent multi-frequency optical source (CMFOS) that can synthesize in the frequency range from gigahertz to terahertz.

Optical link between two optical frequency combs using injection locked laser

We demonstrated the technic of selection of a single comb mode from a mode-locked fiber and linked to the other comb based on electro-optic modulator (EOM). The single comb mode was selected from the fiber laser using a 1.25 GHz spacing FP-filter and injection locking technic. The EOM-comb with spacing of 25 GHz was generated as the side mode of the injection locked comb mode. Using the EOM-comb, we measured the frequency of acetylene stabilized laser of C-band of optical communication. The absolute frequency of the laser was 194 369 569 384.6 kHz and estimated the uncertainty of 3.0×10-12 (k=1).

Improvement of the residual sidemode suppression ratio of injection locked DFB laser using wide-spaced frequcny comb

We present a widely-separated multi-frequency source generated from fiber laser comb using the mode filtering by a Fabry-Pèrot cavity and an injection locking technique. An optical frequency comb was filtered each modes which spacing is 1.5 GHz by a spectral-mode-filtering method by a Fabry-Pérot cavity. The spectral-mode-filtered optical frequency comb which 1.5 GHz is used as the master laser for the injection locking to two distributed-feedback-lasers (DFB, slave lasers). Using these two techniques, single mode selection and power amplification of each comb mode are possible for the generation the widely-separated coherent multi-frequency source from the mode-locked fiber laser.

Optical frequency synthesizer based on selectively injection-locked DFB lasers from a femtosecond fiber laser comb

We present an optical frequency synthesizer based on two DFB lasers that can be selectively injection-locked to a desired single mode among the femtosecond fiber laser comb components.