Electromagnetic fields with photon-number fluctuation reduced below the standard quantum limit have been generated in a constant-currentdriven semiconductor laser. The generation is based on a new principle of high-impedance suppression for pump-amplitude fluctuation in a highly saturated laser oscillator. The observed noise level is 7.3% (31% after correction for detection quantum efficiency) in power below the standard quantum limit in the entire frequency range between 350 and 450 MHz. PACS numbers: 42.50.Dv, 03.65.Bz, 42.65.Bp A number-phase minimum-uncertainty state of the electromagnetic field is mathematically defined as an eigenstate of the operator e n+ie 5, where n is the number operator, 5 the sine operator, and Z a squeezingparameter. ' When I is greater than ln(2(n)) ', the photon-number noise becomes smaller than the standard quantum limit (SQL), (An ) & (n), and the sine-operator (phase) noise becomes larger than the SQL, (AS )/(C) & 1/4(n), while the minimum-uncertainty relationship, (An )(AS ) =(C) /4, is still preserved. Here C is the cosine operator. This "nonclassical state" is analogous to a squeezed state, which is an eigenstate of the operator e a~+ie a2. It features reduced quantum noise in one quadrature, (Aa i ) & -, ', and enhanced quantuin noise in the other quadrature, (Aa2) & 4, while the minimum-uncertainty relationship,(Aai )(Aa2) = -, ', , is still preserved. Here a~and a2 are the two quadrature phase amplitudes. In order to reduce one quadrature noise finally to zero in a squeezed state, an electromagnetic mode must have an infinite photon number. This is because the enhanced quadrature noise consumes the mode energy. This trade-off relationship between quantum noise reduction and required photon number places a limit on the signal-to-noise ratio improvement achievable by a squeezed state. On the other hand, photon-number noise can be reduced to zero without the requirement of an infinite photon number in a number-phase minimumuncertainty state because enhanced phase noise does not consume energy at all. This nonclassical state approaches a photon-number state (or Fock state) as E increases. This generation of a number-phase minimumuncertainty state as well as a squeezed state is of potential importance for information transmission, precision measurement, and atomic spectroscopy. A photonnumber state, specifically, achieves the maximum chan-nel capacity in optical communication, and it also improves the performance of an interferometric gravitywave detector. The observation of quadrature phase squeezing, which is an unmistakable mark for squeezed-state generation, was first reported by Slusher et al. This landmark has been followed by three experimental groups.We have proposed three schemes for generating a number-phase minimum-uncertainty state. These are self-phase modulation in a Kerr medium incorporated with an interferometer, quantum nondemolition measurement incorporated with feedback, ' and pump-amplitude fluctuation suppression in a highly saturated laser oscillator. '' A...