Tunable laser
A laser whose output wavelength can be deliberately and continuously varied across a range, by mechanical, thermal, or electrical control. The standard signal source for WDM testing, coherent communications, and broadband spectroscopy.
A tunable laser is one whose output wavelength can be set, by control of operating parameters, anywhere within a specified tuning range. This distinguishes it from a fixed-wavelength laser (DFB, fiber laser at a specific transition, etc.) whose wavelength is determined at fabrication and varies only weakly with temperature.
Tuning ranges and methods.
| Architecture | Tuning range | Tuning speed | Linewidth |
|---|---|---|---|
| External cavity diode laser (ECDL) — Littrow | 50 – 200 nm (visible/NIR) | nm/s mechanical | 100 kHz |
| ECDL — Littman-Metcalf | 100 – 300 nm | nm/s mechanical | 100 kHz |
| Three-section DBR | 5 – 10 nm continuous | ns – μs | 1 – 10 MHz |
| Sampled-grating DBR (SG-DBR) | 40 – 50 nm (mode-hop) | μs – ms | 100 kHz – 5 MHz |
| Y-branch modulated-grating reflector | 40 – 80 nm | μs | 100 kHz – 5 MHz |
| Ti:sapphire laser | 700 – 1000 nm | nm/s mechanical | 100 kHz – 10 MHz |
| Optical parametric oscillator | 500 nm – 4 μm | mechanical / electronic | 1 – 100 MHz |
| Vernier filter MEMS-tunable VCSEL | 100 nm at 1310/1550 nm | μs – ms | 5 – 50 MHz |
| Tunable Yb fiber laser | 100 nm around 1064 nm | mechanical | 1 – 100 kHz |
Continuous vs mode-hop tuning. Two regimes:
- Continuous (mode-hop-free) tuning: the laser wavelength changes smoothly with the control parameter, with no discrete jumps. Typically 5 – 10 nm range. Required for high-resolution spectroscopy, coherent tracking, narrow-FSR characterization.
- Mode-hop tuning: discrete steps between cavity modes during tuning. Common in SG-DBR and other Vernier-tuned architectures. Acceptable for WDM channel selection (where exact wavelengths within the comb are sufficient) but unsuitable for spectroscopic line scanning.
Wavelength accuracy. Telecom tunable lasers are wavelength-locked to the ITU grid (50 or 100 GHz spacing) using internal etalon-locked feedback control. Typical accuracy: GHz over the full temperature range. Wavemeter-referenced laboratory tunable lasers achieve – pm accuracy.
Standard application areas.
- WDM testing: characterize wavelength-selective filters, ROADMs, AWG demultiplexers
- Component characterization: scan a tunable laser across a wavelength range while monitoring DUT transmission to extract spectral response
- Coherent transceiver local oscillator: matched to the incoming signal wavelength via feedback control
- Tunable Lidar: frequency-modulated continuous-wave (FMCW) lidar uses chirped tunable lasers to measure range and velocity
- Gas-phase spectroscopy: scan across molecular absorption lines for trace gas detection
- Fiber-Bragg-grating sensor interrogation: track FBG center wavelength as strain/temperature sensor
Power. Tunable lasers typically have somewhat lower output power than fixed-wavelength sources at the same drive level, because the wavelength-selective elements introduce intracavity loss. Typical specs:
| Tunable laser type | Output power |
|---|---|
| Telecom SG-DBR | 10 – 100 mW (per channel) |
| Laboratory ECDL (Littrow) | 10 – 50 mW |
| Ti:sapphire | 0.5 – 5 W |
| Tunable fiber laser | 10 mW – 1 W |
| Quantum cascade external cavity (mid-IR) | 50 – 500 mW |
Cost. Telecom-grade tunable lasers (SG-DBR or modulated grating reflector) cost $1000 – $5000 in volume. Laboratory ECDL tunable lasers (Toptica, MOGLabs, Coherent) cost $15k – $50k. Research-grade Ti:sapphire systems cost $50k – $300k.
References: Coldren, Corzine, Mašanović, Diode Lasers and Photonic Integrated Circuits, Ch. 8 (DBR laser arrays); Jayaraman et al., Theory, design, and performance of extended tuning range semiconductor lasers with sampled gratings, IEEE JQE 1993 (the foundational SG-DBR paper).