Thermo Optek Corp. of Waltham, Mass., has retracted its bid to acquire Laser Science Inc. of Newton, Mass. The Thermo Instrument System subsidiary withdrew its registration with the US Securities and Exchange Commission for up to 400,000 shares of common stock that were to be issued as part of the transaction. Asked why the merger was called off, a Thermo Optek spokeswoman told Photonics Spectra only that "the parties to the proposed merger have chosen to terminate negotiations at this time." Thermo Optek develops, builds and markets analytical instruments that use optical spectroscopy technology for chemical analysis. Laser Science manufactures pulsed nitrogen, dye and carbon dioxide lasers and laser-based systems for industrial, scientific, educational and military applications.