Patent Application
20130077646
Embodiments of the present invention relate to laser control and, more particularly, to controlling a laser operating in various temperatures without measuring temperature.
In a typical optical transponder or transceiver, a laser is used as the optical signal source. Lasers tend to be very sensitive to temperature and its slope efficiency and current threshold change with temperature. Typically, laser operation may be controlled by an APC (Automatic Power Control) circuitry to maintain constant optical power over temperature. But in the case of high speed data rate transceivers, such as 10 Gb transceivers (especially for 10 Gb Ethernet and 10 Gb SONET), one would like to maintain constant OMA (Optical Modulation Amplitude) or in the least maintain constant ER (Extension Ratio).
A conventional approach for controlling constant OMA, is to map the temperature dependence of the modulation and perform a table lookup based on the temperature sensed with a microcontroller. This technique requires several hours of calibration and have very high manufacture cost.
Another scheme is to dither the bias and/or the modulation current to detect the slope efficiency of the laser. But the dither signal introduces penalty for the optical signal quality such as mask margin performance and, in cases like Vertical Cavity Surface Emitting Laser (VCSEL) or Fabry Perot Laser (FP), the connector back-reflection prevents the proper use to the dither signal.
Thus it may be beneficial to control laser modulation based on the laser parameters (i.e. no external temperature or dither signal is required).
The foregoing and a better understanding of the present invention may become apparent from the following detailed description of arrangements and example embodiments and the claims when read in connection with the accompanying drawings, all forming a part of the disclosure of this invention. While the foregoing and following written and illustrated disclosure focuses on disclosing arrangements and example embodiments of the invention, it should be clearly understood that the same is by way of illustration and example only and the invention is not limited thereto.
In the following detailed description, like reference numerals and characters may be used to designate identical, corresponding or similar components in differing FIG. drawings. Well-known power/ground connections to integrated circuits (ICs) and other components may not be shown within the figures for simplicity of illustration and discussion. Where specific details are set forth in order to describe example embodiments of the invention, it should be apparent to one skilled in the art that the invention can be practiced without these specific details.
While lasers may be very sensitive to temperature, embodiments relate to maintaining performance and compensating for temperature changes without the need to actually measure temperature thus eliminating the need for temperature sensors and their associated parameter vs. temperature look-up tables or dithering techniques used in the past.
Referring now to
As shown in
In this case for a the laser operating at OMA=0.35 dbm, the linear transfer function may be described as lmod=0.4187*(lb)+7.6785. Thus, laser bias current may now be used to calculate and control the modulation current and compensate tracking error and life aging degradation of the laser. It gives a good status indicator for the laser being controlled and enables on the fly laser calibration. Previous controlling methods based on estimating the laser status by measuring temperatures or dithering the laser operating point are not necessary.
Since this relationship between bias current and modulation current at a constant power appears to apply to lasers across the board, the constants for the linear transfer function of lmod to lb can be obtained at a single temperature; over temperature may not be necessary. However, a single point calibration may not compensate minor errors such as MPD tracking errors and the small temperature instability of the laser driver. For critical applications such as IEEE802.3ae10GBase-SR which requires very small OMA variation over a large temperature range an over temperature calibration may be required. These minor errors may be stored in a lookup table and can be corrected by a microprocessor 28 or similar devices.
The conventional over temperature calibration is very time consuming and very costly, but now with the modulation control based on the lb, there is no longer a need to wait for the temperature to stabilized before calibration and calibrate the laser may be accomplished on the fly (OFC). The calibration time is now limited only by the ramp rate of the environmental chamber.
In addition, by comparing measured bias current (lb) to the bias current at the beginning of life, the ageing of the laser (reduction in slope efficiency) can be tracked and compensated (LAC). This will extend the life of the device and at the same time give an indication of health of the laser and/or determine the End of Life for the laser (ELD).
This invention allows the user to produce optical transceivers (that use DML lasers) with high performance and reliability as well as low cost. The new DLC has demonstrated in SFP+10G SR the highest performance (with only+/−0.15 dB OMA variation over −15C to +75C and fast calibration time (a few of minutes). Further, this technique makes optical transceivers to having high optical eye stability and high quality with low cost. Tradeoff between performance and price is also available (e.g. single point versus over temperature calibration).
The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.
These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.
