1) Field of the Invention
The present invention relates to a semiconductor laser module that is optically modulated by a subcarrier transmission signal that is used as a community antenna television (CATV) signal or in a mobile communication, and an optical transmission apparatus using the semiconductor laser module.
2) Description of the Related Art
Recently, the CATV system and the mobile communication system have archived high-speed transmission in the broadband multimedia communication by using orthogonal frequency division multiplexing (OFDM), which performs a parallel transmission at low speed using a plurality of subcarriers.
On the other hand, in the optical communication, the wavelength division multiplexing (WDM) system is in a practical use, and an optical transmission apparatus that utilizes the characteristics of light to realize a long-distance transmission or a long-distance relay by combining the WDM system with the CATV system or the mobile communication system. This type of optical transmission apparatus is required to have a low distortion and a high carrier-to-noise (CN) ratio.
The optical transmission apparatus is described in, for example, Masato Ishino and et.al., “Low Distortion 1.3 μm Strained-Layer MQW-DFB Laser for AM-SCM Transmission Systems with Large Channel Capacity”, journal of lightwave technology, Vol. 15, No. 11, pp. 2172-2178, Nov. 1997.
The distortion of the optical transmission apparatus can be reduced by lengthening the cavity length of a distributed-feedback (DFB) laser that emits a subcarrier signal light to be modulated by a subcarrier input, and increasing the oscillation output of the DFB laser. However, in this case, the CN ratio degrades in an area where the oscillation output is low, and the low-distortion characteristic and the high CN ratio characteristic cannot be achieved simultaneously.