An optoelectronic equivalent narrowband filter for high resolution optical spectrum analysis

Abstract

To achieve a narrow bandwidth optical filter with a wide swept range for new generation optical spectrum analysis (OSA) of high performance optical sensors, an optoelectronic equivalent narrowband filter (OENF) is investigated and a swept optical filter with bandwidth of several MHz and swept range of several ten nanometers is built using electric filters and a swept laser as local oscillator (LO). The principle of OENF is introduced and analysis of OENF system is presented. Two electric filters are optimized to be RBW filters for high and medium spectral resolution application. Both simulations and experiments are conducted to verify OENF principle and results show that power uncertainty is less than 1.2% and spectral resolution can reach 6 MHz. Then, a real-time wavelength calibration system consisting of HCN gas cell and Fabry-Pérot etalon is proposed to guarantee a wavelength accuracy of ±0.4 pm at C-band and to reduce the influence of phase noise and nonlinear velocity of swept LO. Finally, experiments on OSA of actual spectra of various optical sensors using OENF system are conducted. Experimental results indicate that OENF system has an excellent capacity in analysis of fine spectrum structures.

Effect of Phase Noise on the Frequency Calibration of a Tunable Laser by Heterodyne Signal Filtering

“…Although the analyses described above are based on analog filters, it is easy to be applied in digital filtering, especially for finite impulse response (FIR) filters, the impulse response of which are finite. This means that there is no truncation error if we take the whole impulse response as the analysis interval in (5) and (5a). For digital infinite impulse response (IIR) filters, the impulse response of the filter is truncated similarly to the analog filters.…”

“…The envelope of the impulse response h(t) of a stable and causal linear time-invariant (LTI) system satisfies the following conditions: (a) tends to be 0 when t goes to infinity and (b) equals to 0 when t < 0. Thus we can approximate the result of (5) by rejecting the integral from T to +∞ and write (5) into (5a) as:S…”

“…For example, in light detection and ranging (Li-DAR) systems based on frequency modulated continuous wave (FMCW), the instantaneous optical frequency of the swept laser should be monitored for passive linearization of the tuning speed [1], [2]. In tunable laser spectroscopy, the frequency should be calibrated in order to record the spectral features correctly [3], [4], [5]. Conventional spectrometers such as Fabry-Perot etalon and dispersion gratings, have difficulties in fulfilling both high spectral resolution and wide measuring range.…”

Heterodyne spectrometer sensitivity limit for quantum networking

“…For convenience, the amplified subtracted detector output goes to an electronic spectrum analyzer (ESA, specifically an Agilent N9000A CXA Signal Analyzer). Alternatively, if an ESA is unavailable or a smaller device is desired, an analog-to-digital converter (ADC) connected to a microprocessor could have been used [5] for a more cost-effective and integrated analysis solution.…”

“…Heterodyne spectrometers have the benefit of the frequency resolution being primarily limited only by the local oscillator bandwidth and stability, assuming the post-processing electronics have the required precision. Because of this, heterodyne spectrometers have been demonstrated with resolution < 125 MHz (< 1 pm at 1550 nm) [3][4][5][6][7]. The sensitivity of these heterodyne spectrometers on the other hand has not been as impressive, often not much below -60 dBm [8,9] when using photodiodes.…”