Laser Distance Measurer

Team members: Arya Chowdhury, Liu Sijin, Jonathan Wong

This project aims to build a laser interferometer that measures short distances (i.e., within a room).

Components:

• Laser diode (source)
• Function generator
• Band pass filter
• Photodiode detector
• Amplifier
• (Optional - Arduino board)

Theory / Principle: Light emitted from a laser can be used to measure distances via phase shift detection. Our setup consists of a laser diode (which serves as a light source) with a given frequency (or wavelength) and amplitude. The reflection of the light off a surface serves as a phase shift operation on the incident light. Hence, if we compare the incident light wave to the reflected light wave, we can meausre such a phase shift and therefore calculate the distance.

The detector consists of a photodiode to receive the reflecting light and a circuit to power the photodiode and output the signal. By supplying the dector with a certain voltage, we can construct the circuit as the below diagram and measure the signal via the difference in voltage.

[circuit diagram]

1. Theoretical Background(1.1 laser distance;1.2 phase shift）

This experiment is based on the principle of phase-shift laser ranging, a technique that measures the distance to a target by analyzing the phase difference between a modulated emitted light wave and the corresponding reflected wave. A continuous-wave (CW) laser source is modulated with a sinusoidal or square waveform of known frequency. When the modulated light propagates toward a target and reflects back, the round-trip time introduces a phase delay Δϕ between the emitted and received signals. This phase shift is directly related to the optical path length, and hence the distance, according to the following relation: d=(λ⋅Δϕ)/4π d is the measured distance, λ=c/f is the modulation wavelength, with c being the speed of light and f the modulation frequency, Δϕ is the measured phase difference in radians.

2. Equipmental work

2.1 equipments Laser Diode – 650 nm CW Laser:Provides a stable continuous-wave laser source for modulation and reflection. Function Generator – Tektronix AFG1022:Generates a 10 MHz square wave used to modulate the laser intensity. Photodetector – Hamamatsu S5971:Converts the received optical signal into an electrical signal for phase comparison. DC Power Supply – Keithley 2231A-30-3:Supplies precise voltage and current to the laser module and amplification circuit. Low-Noise Amplifier – High-bandwidth preamplifier (>100 MHz):Amplifies the weak photodetector output while minimizing signal distortion. Lenses – Convex lenses (focal length matched to diode divergence):Used for collimating and focusing the laser beam onto the target. Optical Filter – 650 nm bandpass filter (10 nm FWHM):Eliminates ambient light and improves detection signal-to-noise ratio. Sliding Rail – Precision linear translation stage (millimeter resolution):Allows controlled movement of the reflective target to change optical path length. Oscilloscope – Digital sampling oscilloscope:Captures and compares transmitted and received waveforms to determine phase delay.

2.2 Procedure

2.2.1 Laser Modulation and System Initialization

Configure the function generator (Tektronix AFG1022) to output a 10 MHz square wave with an amplitude of 0–3.1 V (High level: 3.1 V, Low level: 1.0 V, output impedance: 50 Ω) to modulate the laser diode. Use the Keithley 2231A-30-3 DC power supply to provide 5.0 V to the laser driver circuit, with a current limit of 30 mA. Verify the laser beam stability to avoid multimode noise or thermal drift that could distort the modulation waveform.

2.2.2 Optical Alignment and Target Illumination System

Mount the 650 nm laser diode and use a convex lens to collimate and focus the beam onto the reflective target surface. Insert a bandpass optical filter (center wavelength: 650 nm, bandwidth: 10 nm) into the return path to suppress ambient light interference. Fix the reflective target (diffuse or specular surface) on a precision linear translation stage, ensuring proper alignment for beam return to the detector at all positions.

2.2.3 Detection and Amplification Circuit Configuration

Use a Hamamatsu S5971 silicon photodiode to detect the reflected light signal.Connect the detector output to a matched high-speed preamplifier (bandwidth >100 MHz, gain ~10³–10⁵) to amplify the signal linearly without saturation.Connect the amplifier output to Channel 2 of a digital oscilloscope; connect the reference signal from the function generator to Channel 1 as the modulation phase reference.

2.2.4 Static Testing: Time Delay vs. Distance Mapping

Fix the reflective target at several predefined positions along the translation stage (e.g., every 5 cm) and record the waveform signals from both channels at each position.Use the oscilloscope’s cursor measurement function to determine the relative time delay Δt between the modulation reference signal (Channel 1) and the return signal (Channel 2).Correlate the measured delay values with corresponding distances to establish the system’s delay–distance response curve.

2.2.5 Dynamic Scan Testing: Continuous Distance Response Verification

Move the target along the sliding rail at a constant speed, and continuously record the time delay of the return signal at multiple positions. Configure the oscilloscope trigger conditions to ensure stable waveform acquisition. Plot the variation of time delay as a function of target position to analyze the linearity and temporal resolution of the system.

2.3 Measurement techniques

3. Result（3.1 graph with/out mirror 3.2 data theory/exp）

4. Challenges and Mitigation Lack of focusing lens on detector, reduced coupling efficiency.Added plano-convex lens increased SNR by 8 dB. Oscilloscope trigger jitter. Mitigated by using external trigger from generator TTL output. Potential amplifier saturation. Adjusted gain to 40 dB; further reduction lost signal at long range. Background light fluctuation.Filter plus lab-lights-off reduced baseline drift to <1 mV.