The analysis of optical spectra – emission or absorption – has been arguably the most powerful approach for discovering and understanding matters. The invention and development of many kinds of spectrometers have equipped us with versatile yet ultra-sensitive diagnostic tools for trace gas detection, isotope analysis, and resolving hyperfine structures of atoms and molecules. With proliferating data and information, urgent and demanding requirements have been placed today on spectrum analysis with ever-increasing spectral bandwidth and frequency resolution. These requirements are especially stringent for broadband laser sources that carry massive information, and for dispersive devices used in information processing systems. In addition, spectrum analyzers are expected to probe the device’s phase response where extra information is encoded.
Here we demonstrate a novel vector spectrum analyzer (VSA) that is capable of characterizing passive devices and active laser sources in one setup. Such a dual-mode VSA can measure loss, phase response and dispersion properties of passive devices. It also can coherently map a broadband laser spectrum into the RF domain. The VSA features a bandwidth of 55.1 THz (1260 to 1640 nm), frequency resolution of 471 kHz, and dynamic range of 56 dB. Meanwhile, our fiber-based VSA is compact and robust. It requires neither high-speed modulators and photodetectors, nor any active feedback control. Finally, we successfully employ our VSA for applications including characterization of integrated dispersive waveguides, mapping frequency comb spectra, and coherent light detection and ranging (LiDAR). Our VSA presents an innovative approach for device analysis and laser spectroscopy, and can play a critical role in future photonic systems and applications for sensing, communication, imaging, and quantum information processing.
Figure 1.Principle and architecture of the vector spectrum analyzer (VSA). a. The principle of our VSA is based on a chirping CW laser that is sent to and transmits through a device under test (DUT). The DUT can be either a passive device or a broadband laser source. The transmission spectrum of the chirping laser through the DUT is a time-domain trace. For passive devices, this trace carries the information of the DUT’s loss, phase and dispersion over the chirp bandwidth. For active laser sources, the chirping laser beats progressively with different frequency components of the optical spectrum, thus analyzing the beat signal in the RF domain allows extraction of the spectral information. In short, the chirping laser coherently maps the DUT’s frequency-domain response into the time domain. Critical to this frequency-time mapping is precise and accurate calibration of the instantaneous laser frequency during chirping. This requires to refer the chirping laser to a “frequency ruler”. b. Experimental setup. The frequency-calibration unit here is a phase-stable fiber cavity of 55.58 MHz FSR. The chirping laser unit can be a single laser, or multiple lasers that are bandwidth-cascaded together. The latter allows the extension of the full spectral bandwidth by seamless stitching of individual laser traces into one trace. PD, photodetector. OSC, oscilloscope.
The principle of our VSA is illustrated in Fig. 1a. A continuous-wave (CW), widely chirping laser is sent to and transmits through a device under test (DUT) that can be either a passive device or a laser source. During laser chirping, for passive devices, the frequency-dependent linear transfer function (LTF) containing the DUT’s loss and phase information is photodetected and recorded. For laser sources, the chirping laser beats progressively with different frequency components of the optical spectrum, and the beatnote signal is digitally recorded in the RF domain using a narrow-band-pass filter. In both cases, the VSA outputs a time-domain trace, with each data point corresponding to the DUT’s instantaneous response at a particular frequency during laser chirping. In short, the chirping laser coherently maps the DUT’s frequency-domain response into the time domain. Since the laser cannot chirp perfectly linearly, critical to this frequency-time mapping is precise and accurate calibration of the instantaneous laser frequency at any given time. This requires to refer the chirping laser to a calibrated “frequency ruler”.
Following this principle, we construct the setup as shown in Fig. 1b. A widely tunable, mode-hop-free, external-cavity diode laser (ECDL, Santec TSL) is used as the chirping laser. Cascading multiple ECDLs covering different spectral ranges allows the extension of full spectral bandwidth, which is 1260 to 1640 nm (55.1 THz) in our VSA with three ECDLs.
The ECDL’s CW output is split into two branches. One branch is sent to the DUT and the other is sent to a frequency-calibration unit. Such frequency calibration involves relative- (i.e. the frequency change relative to the starting laser frequency) and absolute-frequency- calibration (i.e. accurately measured starting laser frequency), The absolute-frequency calibration is performed by referring to a built-in wavelength meter with an accuracy of 200 MHz.
