a) VCSELs are more efficient, have a circular beam profile, operate in a single mode, and are smaller than edge-emitting lasers.
b) The edge-emitting laser supports approximately 1000 modes, while the VCSEL supports approximately 480 modes.
a) The advantages of a vertical cavity surface-emitting laser (VCSEL) over a standard edge-emitting laser diode are:
Higher efficiency: VCSELs are more efficient than edge-emitting lasers, meaning they convert more electrical power into optical power. This is because VCSELs have a lower threshold current and a higher differential quantum efficiency.
Circular beam profile: VCSELs emit a circular beam profile, which is easier to couple into optical fibers and other optical components. Edge-emitting lasers emit an elliptical beam profile, which can be more difficult to couple.
Single-mode operation: VCSELs typically operate in a single mode, meaning they emit light at a single wavelength. Edge-emitting lasers can operate in multiple modes, which can lead to spectral broadening and other problems.
Smaller size: VCSELs are typically smaller than edge-emitting lasers, making them more suitable for use in compact devices.
b) To estimate the number of modes that each laser supports, we can use the following equation:
N = (λ/Δλ) * (L/λ)
where:
N is the number of modes
λ is the wavelength of the laser light
Δλ is the FWHM gain bandwidth of the semiconductor material
L is the length of the laser cavity
For the edge-emitting laser, we have the following:
λ = 11 μm
Δλ = 5 THz
L = 250 μm
Plugging these values into the equation, we get:
N = (11 μm / (5 THz * λg)) * (250 μm / 11 μm)
N ≈ 1000
For the VCSEL, we have the following:
λ = 10 μm
Δλ = 5 THz
L = 120 μm
Plugging these values into the equation, we get:
N = (10 μm / (5 THz * λg)) * (120 μm / 10 μm)
N ≈ 480
Therefore, the edge-emitting laser supports approximately 1000 modes, while the VCSEL supports approximately 480 modes.