  The sixth-generation (6G) network is expected to achieve global coverage
based on the space-air-ground integrated network, and the latest satellite
network will play an important role in it. The introduction of inter-satellite
links (ISLs) can significantly improve the throughput of the satellite network,
and recently gets lots of attention from both academia and industry. In this
paper, we illustrate the advantages of using the laser for ISLs due to its
longer communication distance, higher data speed, and stronger security.
Specifically, space-borne laser terminals with the acquisition, pointing and
tracking mechanism which realize long-distance communication are illustrated,
advanced modulation and multiplexing modes that make high communication rates
possible are introduced, and the security of ISLs ensured by the
characteristics of both laser and the optical channel is also analyzed.
Moreover, some open issues such as advanced optical beam steering, routing and
scheduling algorithm, and integrated sensing and communication are discussed to
direct future research.
