7.3 Laser diode characteristics and performance metrics

Laser diodes are crucial in optoelectronics, converting electrical energy into light. This section dives into their key characteristics, including light output, spectral properties, and modulation capabilities. Understanding these metrics is essential for optimizing laser diode performance in various applications.

We'll explore the L-I curve, threshold current density, and efficiency metrics. We'll also examine spectral linewidth, temperature dependence, and beam properties. These factors impact laser diode performance in real-world scenarios, from telecommunications to high-power industrial applications.

Light Output Characteristics

L-I Curve and Threshold Current Density

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• Light-current (L-I) curve plots the output optical power as a function of the input current
  • Below the threshold current, the laser diode acts as an LED with low output power
  • Above the threshold current, the output power increases linearly with the input current
• Threshold current density $J_{th}$ is the minimum current density required for lasing action to occur
  • Depends on factors such as the active layer material, cavity length, and mirror reflectivity
  • Lower $J_{th}$ leads to higher efficiency and lower power consumption

Slope Efficiency and Output Power

• Slope efficiency $\eta_s$ is the ratio of the change in output optical power to the change in input current above the threshold
  • Measured in W/A and represents how efficiently the laser diode converts additional current into light
  • Higher $\eta_s$ indicates better performance and is desirable for high-power applications
• Output power is the maximum optical power that the laser diode can emit
  • Limited by factors such as the maximum current density the device can withstand and thermal effects
  • High-power laser diodes (HPLDs) can have output powers ranging from a few watts to several hundred watts

Wall-Plug Efficiency

• Wall-plug efficiency $\eta_{wp}$ is the ratio of the output optical power to the input electrical power
  • Measures the overall energy conversion efficiency of the laser diode
  • Depends on factors such as the slope efficiency, threshold current, and operating voltage
  • Typical values range from 30% to 70% for high-efficiency laser diodes (GaAs, InGaAs)

Spectral Properties

Spectral Linewidth

• Spectral linewidth $\Delta\lambda$ is the full width at half maximum (FWHM) of the laser diode's emission spectrum
  • Determines the laser's ability to resolve fine spectral features and its suitability for applications such as spectroscopy and fiber optic communication
  • Affected by factors such as the active layer material, cavity design, and operating conditions
  • Single-mode laser diodes (DFB, DBR) can achieve linewidths below 1 MHz (< 0.01 pm)

Temperature Dependence

• The emission wavelength of a laser diode shifts with temperature at a rate of about 0.1 nm/°C
  • Caused by changes in the bandgap energy and refractive index of the active layer material
  • Requires temperature stabilization for applications that demand precise wavelength control
• The threshold current and slope efficiency also vary with temperature
  • Higher temperatures lead to increased non-radiative recombination and reduced efficiency
  • Temperature-stabilized laser diode packages (butterfly, TO-can) often include a thermoelectric cooler (TEC) and thermistor for active temperature control

Modulation and Beam Properties

Modulation Bandwidth

• Modulation bandwidth $f_{3dB}$ is the maximum frequency at which the laser diode's output power can be modulated to 50% of its unmodulated value
  • Determines the maximum data rate for optical communication systems
  • Depends on factors such as the carrier lifetime, cavity photon lifetime, and parasitic capacitance
  • High-speed laser diodes (VCSELs, DFBs) can achieve modulation bandwidths exceeding 50 GHz

Beam Divergence and Far-Field Pattern

• Beam divergence is the angular spread of the laser diode's output beam
  • Characterized by the full-width at half-maximum (FWHM) angles $\theta_{\parallel}$ and $\theta_{\perp}$ in the planes parallel and perpendicular to the junction
  • Typical values range from 10° to 30° for edge-emitting laser diodes and 15° to 50° for VCSELs
• Far-field pattern describes the spatial intensity distribution of the laser diode's output beam at a large distance from the source
  • Affected by the waveguide design, emission aperture, and beam-shaping optics (lenses, prisms)
  • Single-mode laser diodes (DFB, DBR) exhibit a nearly Gaussian far-field pattern, while multi-mode devices (FP) have a more complex profile