How Fiber Optic Gyroscopes Work
The fiber optic gyroscope is composed of a light source, a detector, a coupler, a MIOC and an optical fiber coil. It is essentially a coil interferometer based on the Sagnac effect, that is, after the light waves propagating in the opposite direction along the closed optical path return to the starting point and interfere, the phase of the interference signal The difference is proportional to the input angular velocity of the sensitive axis of the closed optical path. From the analysis of the optical path schematic diagram, the light beam emitted from the SLD light source is divided into two beams by the coupler and the MIOC. The other beam is split by the MIOC and then enters the fiber coil and propagates counterclockwise, exits the fiber coil and returns to the upper arm of the MIOC. The two beams of light from different channels generate interference signals after rounds, and the intensity changes of the interference signals are detected by the detector, so as to obtain the corresponding angular velocity changes.
The light source plays a very important role as one of the core optical components in the fiber optic gyroscope, which is to provide the fiber optic gyroscope with the appropriate optical signal required to produce the Sagnac effect. The light source used in the fiber optic gyroscope requires high reliability, small size, and long life. At the same time, it also requires the light source to have high power, low spatial coherence (broad spectrum) and weak time correlation (wavelength/power long-term stability) and so on.
Broadband light source
There are two main types of broadband light sources currently used in fiber optic gyroscopes: superluminescent light-emitting diode (SLD) light sources and amplified spontaneous emission (ASE) light sources. The former is a semiconductor light source in which photon spontaneous emission is amplified by one-way excitation. Because SLD has one-way amplification, the output power can reach the order of mW, and it has narrow beam divergence angle, wide output spectrum and high stable pigtail output light. power, so that the interference signal with a higher signal-to-noise ratio can be obtained. The ASE light source is to amplify spontaneous emission light, with higher output power, wider spectrum, higher wavelength stability and power stability, and is mostly used in high-precision fiber optic gyroscope systems. The following are typical spectra of ASE light sources and SLD light sources:
ASE Rectangular Spectrum SLD Gaussian Spectrum
Fig. 2 typical spectrum
Introduction to SLD Light Sources
The schematic diagram of the internal structure of the SLD light source is shown in Figure 3. It consists of SLD die, thermistor, heat sink, semiconductor cooler (TEC), pigtail, housing and other components. The TEC is used to control and stabilize the die temperature of the light source, the thermistor is used to sensitively change the die temperature of the light source, and the heat sink has a large thermal conductivity and is used for heat dissipation.
Fig. 3 internal structure of SLD light source
In the quantum well SLD die, under the action of the forward injection current, the holes from the p-layer and the electrons from the n-layer combine in the light-emitting layer to emit photons. Excited amplification produces laser light, which narrows the spectrum and divergence angle, and increases the modulation bandwidth. The working principle of the SLD light source component is: under the drive of the injected current, the heat generated in the SLD operation is exported through the heat sink. In order to stabilize the output optical power and wavelength, the temperature is controlled by the TEC. The light wave output by the die is coupled out through the pigtail, and the optical fiber is metallized to fix the relative position between the optical fiber and the chip through metal welding to ensure the stability of the output optical power. The output power of the SLD will affect the output signal-to-noise ratio of the photodetector in the FOG.
Introduction to ASE light source:
The ASE (Amplified Spontaneous Emission) light source is specially designed for production and laboratory experiments. The main part of the light source is a gain medium erbium-doped fiber and a high-performance pump laser. The unique ATC and APC circuits ensure the stability of the output power by controlling the output of the pump laser. By adjusting the APC, the output power can be adjusted within a certain range.
The relationship between the main parameters of the light source and the performance of the fiber optic gyroscope
Effect of light source output power on gyro noise
The shot noise of the detector limits the resolution of the FOG to measure the angular velocity of the carrier. Therefore, in order to improve the signal-to-noise ratio of the FOG readout signal, the output optical power of the FOG light source must be increased. The fiber optic gyroscope measures the Sagnac phase difference generated by the rotation. The measurement of the phase difference is realized by detecting the change of the optical power and according to the relationship between the phase difference and the optical power, so it is limited by the measurement limit of the optical power. In the interferometric fiber optic gyroscope, the random walk caused by shot noise is inversely proportional to the square root of the optical power returning to the detector, and the optical power returning to the detector is related to the total optical path loss and the output power of the light source. Therefore, it can be seen that the output optical power of the light source has a great influence on the performance of the fiber optic gyroscope. The output optical power of the SLD light source and the ASE light source can reach the mW level, which fully meets the requirements of the navigation-grade fiber optic gyroscope for the light source.
