available units in accordance with the present invention the filter is removed or bypassed and the transducer is excited by applying a low amplitude drive signal at one of the mechanical resonance frequencies to provide extremely rapid oscillatory length changes thus sweeping the wavelength of particular modes to fill the gaps between the discrete longitudinal modes of the laser as an example of the operation of this invention consider a broadband cw standing wave dye laser as shown in fig1 having a resonant cavity length of 75 cm and normally operating with an output centered about a wavelength 460 nm and a longitudinal mode spacing of about 200 mhz if the pzt 27 is resonantly driven at a longitudinal mechanical resonance of for example 350 khz and the amplitude of the driving voltage is 3 or 4 volts about a 5 volt dc offset to prevent a negative drive voltage cavity length changes of u00b1 5 to u00b1 1o u03bcm can be realized at the resonant frequency of 350 khz since in this example a 5 u03bcm cavity length change corresponds to an optical frequency change of u00b1 2 ghz i e u00b1 10 longitudinal cavity modes of 200 mhz the wavelength of a particular cavity mode has been oscillated by u00b1 2 ghz thus an optical frequency gap of 200 mhz between normal stationary longitudinal modes is completely covered in a.