Directly Modulated Laser With Variable Light Reflector
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Solution Overview
Problem
Directly modulated lasers (DMLs) face limitations in modulation speed due to the inherent properties of their gain medium, leading to frequency shifts and reduced operational lifetime, as well as electrical and thermal stress.
Innovation Solution
A directly modulated semiconductor laser with a variable end reflector using a lightwave circuit and optical interferometers, where the transmittance and reflectance of the reflector are modulated by an electrical RF signal, allowing for high-speed phase and amplitude modulation of the optical output.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the laser's injection current is modulated to generate a modulated optical signal, then the optical output can be modulated, but the modulation speed is limited by the inherent properties of the gain medium
Solution Approach 1:
The patent introduces an external modulator as an intermediary device between the laser and the output. The laser operates continuously at a fixed current to avoid stress on the gain medium, while the external modulator imprints the modulation signal onto the continuous optical output, thereby achieving high-speed modulation without compromising the laser's operational lifetime
Solution Approach 2:
The modulation function is extracted from the laser's gain medium and transferred to a separate external modulator. This separation allows the laser to operate in a relaxed continuous-wave mode while the modulator handles the high-speed signal imposition, resolving the contradiction between modulation speed and gain medium stress
2Ease of operation
If the injection current is changed to modulate the optical output, then modulation is achieved, but electrical and thermal stress is imposed on the gain medium
Solution Approach 1:
An external modulator serves as a mediator that imposes modulation on the continuous-wave optical output without requiring current changes in the laser. This eliminates electrical and thermal stress on the gain medium while preserving full modulation capability through the external device
Solution Approach 2:
Instead of modulating the laser current to achieve modulation (conventional approach), the patent inverts the approach by keeping the current constant and using an external device to modulate the optical output. This inversion eliminates the harmful stress while maintaining modulation functionality
3Productivity
If the laser operates at high modulation speeds, then communication bandwidth is increased, but frequency shifts and transients occur due to gain medium properties
Solution Approach 1:
The external modulator acts as an intermediary that imprints modulation signals onto the stable continuous-wave optical output. This process occurs after the light is generated, avoiding interaction with the gain medium's relaxation oscillations and thus eliminating frequency shifts and transients while enabling high communication bandwidth
Solution Approach 2:
The laser operates in continuous-wave mode as a preliminary step, generating a stable optical carrier before modulation is applied by the external modulator. This preliminary stable generation prevents the formation of frequency shifts and transients that would occur if modulation were applied through current changes
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enables modulation speeds greater than 10 GHz, overcoming the limitations of the gain medium and reducing stress on the laser, thereby extending its operational lifetime and improving performance.
Implementation Method 1
a waveguide section configured to modulate the phase of an optical beam passing therethrough in response to an electrical radio-frequency drive signal
Implementation Method 2
the end reflector can be implemented using a lightwave circuit in which optical waveguides are arranged to form an optical interferometer
Data Source
AI summary
A directly modulated semiconductor laser whose optical output can be modulated by varying the transmittance of an end reflector of the laser cavity. In an example embodiment, the end reflector can be implemented using a lightwave circuit in which optical waveguides are arranged to form an optical interferometer. At least one of the optical waveguides may include a waveguide section configured to modulate the phase of an optical beam passing therethrough in response to an electrical radio-frequency drive signal in a manner that causes the transmittance and reflectance of the end reflector to be modulated accordingly. Advantageously, relatively high (e.g., >10 GHz) phase and/or amplitude modulation speeds of the optical output can be achieved in this manner to circumvent the inherent modulation-speed limitations of the laser's gain medium.