This video provides an in-depth teardown and comprehensive characterization of vintage HP lightwave transmitter and receiver modules. The teardown reveals the internal architecture, focusing on the key electro-optic components. The transmitter module is built around a 1310nm Distributed Feedback (DFB) laser stabilized by a thermoelectric cooler, featuring a manually tuned RF matching network for direct current modulation. The receiver uses a Gradient Index (GRIN) lens to focus light onto a PIN photodiode, followed by a multi-stage amplifier chain.
Using a combination of Siglent and Keysight test equipment, the end-to-end optical link is thoroughly characterized. Measurements confirm the laser's wavelength, the link's frequency response beyond 6 GHz, and its high linearity through two-tone intermodulation tests. The system's versatility is demonstrated by successfully transmitting various signals, including a 256-QAM "radio over fiber" signal, a clean 625 Mbit/s digital NRZ signal, and an impressive 24 Gbit/s 64-QAM signal, showcasing the remarkable performance of these legacy components.
A Deep Dive into Vintage HP Optical Modules: Teardown and Characterization
00:00:53 - Transmitter Architecture: The transmitter's core is a 1310nm Distributed Feedback (DFB) laser. Its optical power and temperature are actively stabilized using a back-face photodiode and a thermoelectric cooler (TEC) to ensure a stable output. Modulation is achieved by directly varying the current through the laser diode, which modulates the light's intensity.
00:06:11 - Receiver Architecture: The receiver converts the optical signal back to an electrical one. It uses a Gradient Index (GRIN) lens, which has a flat end, to focus the incoming light onto a PIN diode. The small electrical signal from the PIN diode is then amplified by a multi-stage amplifier.
00:11:32 - Transmitter Teardown & Manual Tuning: A detailed look inside the transmitter's optical module reveals a complex RF matching network. This network uses manually bent metal strips connected to a stripline to fine-tune the impedance and flatten the frequency response for driving the laser diode.
00:14:37 - Microscopic Examination: Under the microscope, the video shows the precise, albeit crude, alignment of the optical fiber to the DFB laser chip. The back-face photodetector used for the power feedback loop is also visible, sitting on a thermally controlled "island" above the TEC.
00:20:18 - Receiver Teardown & Amplifiers: The receiver module teardown shows the PIN photodiode at the center, followed by a chain of amplifiers. The amplifiers use techniques like inductive peaking, created with carefully shaped wire bonds, to boost the high-frequency response.
00:24:54 - Optical Spectrum Verification: Using an optical spectrum analyzer, the transmitter's output is confirmed to be at 1320.9 nm (very close to the specified 1310 nm) with about 2.5 dBm of power. The spectrum is clean, indicating the laser is functioning correctly.
00:27:27 - End-to-End Frequency Response: Measuring the S-parameters of the complete link (electrical-in to electrical-out) with a Vector Network Analyzer shows a relatively flat frequency response up to and beyond its 6 GHz specification, with a total loss of about 8-9 dB.
00:29:33 - Linearity Verification: The link's linearity is tested by inserting a variable optical attenuator. A 3 dB optical loss results in a 6 dB electrical loss, which is the theoretically correct relationship for a direct detection system, confirming it operates linearly.
00:34:12 - Two-Tone Intermodulation Test: The link's linearity is further tested using a two-tone signal. The resulting intermodulation distortion is found to be exceptionally low, making the link suitable for transmitting complex analog signals, a concept known as "radio over fiber."
00:39:01 - Radio Over Fiber Demonstration: A 256-QAM signal (256 Mbit/s) is successfully transmitted through the optical link. The signal is demodulated live, showing a clean constellation and proving the system's ability to act as a transparent transport for complex RF signals.
00:42:01 - Digital NRZ Performance: The link is tested with a standard 625 Mbit/s digital NRZ signal. The resulting eye diagram on an oscilloscope is wide open with minimal noise and jitter, indicating excellent performance for baseband digital communication.
00:45:43 - Pushing the Limits with High-Speed Data: Using high-end test equipment, the link is pushed to its limit by transmitting a 4 Gbaud 64-QAM signal, which is equivalent to 24 Gbit/s. The system successfully handles this high data rate, demonstrating the incredible bandwidth potential of the vintage fiber optic link.