Performance Evaluation of Mid-IR Spectrometers by Using a Mid-IR Tunable Optical Parametric Oscillator

중적외선 광 파라메트릭 발진기를 이용한 중적외선 분광기 성능 평가

Abstract

We have used a mid-IR (mid-infrared) continuous-wave (cw) optical parametric oscillator (OPO), developed previously and described in Ref. 12, to build a performance-evaluation setup for a mid-IR spectrometer. The used CW OPO had a wavelength tuning range of $ 2.5-3.6{\mu}m$ using a pump laser with a wavelength of 1064 nm and a fan-out MgO-doped periodically poled lithium niobate (MgO:PPLN) nonlinear crystal in a concentric cavity design. The OPO was combined with a near-IR integrating sphere and a Fourier-transform IR optical spectrum analyzer to build a performance-evaluation setup for mid-IR spectrometers. We applied this performance-evaluation setup to evaluating a mid-IR spectrometer developed domestically, and demonstrated the capability of evaluating the performance, such as spectral resolution, signal-to-noise ratio, spectral stray light, and so on, based on this setup.

기존에 개발되었던 광 파라메트릭 발진기를 이용해 중적외선 분광기의 성능 평가 장치를 구축하였다. 사용된 중적외선 광 파라메트릭 발진기는 파장 1064 nm의 연속파 레이저로 펌핑되는 fan-out형 MgO-doped periodically poled lithium niobate (MgO:PPLN) 비선형 결정을 사용하여 파장가변 영역이$ 2.5{\sim}3.6{\mu}m$인 공중심 공진기 구조를 가지고 있다. 이 광 파라메트릭 발진기에 중적외선용 적분구 및 푸리에 변환 적외선 광 스펙트럼 분석기를 결합하여 중적외선 분광기 성능평가 장치를 구축하였다. 구축된 평가장치를 국내에서 개발한 중적외선용 분광기에 적용, 성능 평가를 진행함으로써 본 평가 장치를 분광기의 파장 분해능, 신호대잡음비, 분광 떠돌이광 등의 성능을 평가하는데 활용할 수 있음을 보였다.

Fig. 1. Schematic diagram of the OPO setup by using MgO:PPLN single crystal and a DPSS pump laser (Nd:YAG) for generating continuous-wave output in the mid-IR range (BS, beam splitter; CW DPSS, continuous-wave diode pumped solid state; HR@1064, high-reflectivity mirror at 1064 nm; HWP, half-wave plate; M, mirror; DM, dichroic mirror).

Fig. 3. Throughput obtained by 2-dimensional scanning of the parallel laser beam of light source at 4 different wavelengths.

Fig. 4. Wavelength dependence of the relative throughput obtained from the ratio of the two output measurements.

Fig. 5. The output of the mid-IR spectrometer for the input beam at the wavelength of 3.1 μm as a function of wavelength (vertical axis) and spatial coordinate (horizontal axis).

Fig. 2. (a) Schematic diagram showing the alignment of mid-IR light sources, the integrating sphere, the FTIR OSA and the mid-IR spectrometer. (b) A photograph showing the evaluation optical setup for the mid-IR spectrometer. (LIRS, Lunar InfraRed Spectrometer).

Table 1. Relative uncertainty and relative standard uncertainty of the individual components at 4 different wavelengths