Brillouin fiber laser spectrometer
US20250334450A1
Abstract
A method, wherein an optical input signal is received. The optical input signal includes an optical signal power and an input optical spectrum. The optical input signal is split into a first optical replica of the optical input signal and a second optical replica of the optical input signal. The first optical replica of the optical input signal is transmitted through a fiber laser cavity. A portion of the at least one lasing mode is transmitted from the fiber laser cavity to an optical heterodyne receiver, and the second optical replica of the optical input signal is transmitted to the optical heterodyne receiver. An electrical output signal including an output electrical spectrum is generated. The output electrical spectrum includes a compressed replica of the input optical spectrum. A measurement of the input optical spectrum is determined based on a respective Brillouin frequency shift and at least one input frequency.
Description (excerpt)
CROSS-REFERENCE TO RELATED APPLICATIONS This Application claims the benefit of U.S. Provisional Application Ser. No. 63/417,345 filed on 19 Oct. 2022, the entirety of which is incorporated herein by reference. Also, this Application is a Divisional of, and claims priority to, U.S. patent application Ser. No. 18/243,728 filed on 8 Sep. 2023, the entirety of which is incorporated herein by reference. FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT The United States Government has ownership rights in this invention. Licensing inquiries may be directed to Office of Technology Transfer, US Naval Research Laboratory, Code 1004, Washington, DC 20375, USA; +1.202.767.7230; techtran@nrl.navy.mil, referencing NC 211313-US2. BACKGROUND OF THE INVENTION Field of the Invention This invention relates in general to an optical spectrum analyzer, and in particular to a fiber laser spectrometer. Description of the Related Art Prior art spectrum analyzers either function by scanning or spreading the spectrum in time or space with the spectrum then measured in a single shot. On the one hand, prior art scanning analyzers such as grating-based scanning spectrometers, Fourier transform infrared spectrometers (“FTIRs”), or Brillouin optical spectrum analyzers (“BOSAs”) can measure at high resolution, but require slow scans to do so. On the other hand, prior art single-shot measurement devices, such as grating-based spectrographs or recently developed Rayleigh fiber backscattering based spectrometers, can capture the entire spectrum of interest, but have trade-offs between resolution and measurement range. Additionally, there exists a wide range of wavemeter architectures capable of performing high precision measurements with short acquisition times and large measurement ranges, such as those based on interferometry. However, these wavemeter architecture systems are capable of measuring only a single input frequency. BRIEF SUMMARY OF THE INVENTION An embodiment of the invention includes an apparatus and/or a method that enables high speed high precision spectroscopy measurements encompassing the entire telecom C-band while capturing spectra at high speed. An embodiment of the invention does not require a high precision laser for reference. An embodiment of the invention includes an optical spectrum analyzer that relies on the linear relationship between optical frequency and the Brillouin frequency shift (BFS) in optical fiber. The Brillouin frequency, which is well characterized in optical fiber, changes by ˜55 kHz per GHz of optical frequency. To measure the Brillouin frequency shift, the inventors use the unknown signal under test as a “pump” to excite Brillouin lasing in a fiber ring cavity. The cavity resonance is broken by pulsing an intensity modulator within the cavity at a repetition period matching the round-trip time in the cavity. This results in a pulsed lasing mode downshifted from the pump frequency to the peak of the Brillouin gain spectrum. This Brillouin frequency shift is proportional to the initial pump frequency with a slope in optical fiber of ˜0.55×10 −4 Hz shift per Hz of pump frequency for a pump in the range of 1550 nm. The inventors then measure the relative frequency between this Brillouin lasing mode and the original signal under test via heterodyne detection-providing a self-referenced measurement of the BFS and, thus, the absolute frequency of the signal under test. This approach effectively compresses a wideband laser input (C-band) to an RF spectrum easily managed by typical analog to digital converters (˜500 MHz) without requiring scanning or a reference laser. The difficulty of this approach then becomes measuring the BFS to a very high precision to overcome the 104 conversion penalty. This approach draws from the inventors' earlier work concerning distributed Brillouin fiber laser sensors, such as described in [1], which is incorporated herein by reference. In that work, the inventors were able to reduce the uncertainty of our BFS to ˜200 Hz/√Hz for a 400 m test fiber. In this context, such precision would result in ˜ 4 MHz/VHz error in the determination of the unknown signal under test. Embodiments of the invention, for example, combine high spectral resolution (˜20 MHz or ˜0.16 pm in a 200 μs measurement) with a wide bandwidth (˜6 THz or 48 nm—covering the entire telecom C band) and a fast measurement rate (˜10 kHz). An entire system according to an embodiment of the invention is, for example, constructed using off-the-shelf fiber-coupled components, providing a compact and robust approach to optical spectrum analysis. An embodiment of the invention includes an apparatus. The apparatus includes a spectrometer receiving an optical input signal. The optical input signal includes an input optical spectrum. The input optical spectrum includes at least one input frequency.
Filing details
- Inventors
- Joseph B. Murray
- Assignee
- The Government Of The United States Of America, As Represented By The Secretary …
- Filed
- Jul 1, 2025
- Granted
- Application pending
Bibliographic data and excerpted text sourced from Google Patents (public record) as part of IP TechMatch's current-filings monitor. This filing is not part of the 2019 historical archive. For the authoritative full text, drawings, and legal status, see the source links above or consult USPTO records directly.